During the summer holidays of 1960 to 1963 a great many localities of Miocene and Pliocene mammals were discovered in the Southern part of Zaragoza province (Spain). Remains of both large and small mammals were found, the latter by sieving clays and marls. This small fauna consists mainly of Cricetodontinae, Gliridae, Sciuridae, Lagomorpha and Insectivora. The object of the present study is the Cricetodontinae. Other families of the same localities will be described by H. de Bruyn of the State University of Utrecht, Netherlands. The localities are situated along the S.W. border of the Calatayud-Teruel basin. Apart from small local faulting and folding along the margin of the basin the sediments are undisturbed. This circumstance and the nearly continuous exposures made it possible to determine the relative stratigraphic position of the sites. With the stratigraphic succession as a directive it proved to be possible to follow the evolution of the Cricetodontinae step by step. The Cricetodon material found in the region falls into four groups. Within each group species are closely related. 1. The group minor (based on Cricetodon minor LARTET, 1851) comprises small species with elongated M1. The oldest member of this group is Cricetodon minor primitivus n. ssp. from the Valtorres locality near Calatayud. This form develops to C. collongensis MEIN, 1958, from Vieux-Collonges (France). The author considers this to be a subspecies of C. minor LARTET. Cricetodon minor collongensis was found in several Spanish localities. The evolutionary development of this form appears to be C. minor minor LARTET on the one side and C. gregarius SCHAUB, 1925 on the other. Representatives of both branches were found in the Calatayud-Daroca region. In the course of time members of the gregarius branch grow gradually larger, whereas in the minor branch they stay of about the same size. 2. The group larteti (based on Cricetodon larteti SCHAUB, 1925) comprises species of medium size. The molars are short and broad, squarishly built, and show definite reduction. The early and middle Miocene ancestors of Cricetodon larteti were now discovered in Spain, namely C. koenigswaldi n.sp. and C. darocensis n.sp. The most notable feature of the evolutionary trend in this group is a gradual increase in size. 3. The group affinis (based on Cricetodon affinis SCHAUB, 1925) comprises species of small and medium size. The molars are squat and show little reduction. C. brevis SCHAUB is synonymous with C. affinis. The type specimen of C. brevis from La Grive-St. Alban (France) is considered to be a small specimen of C. affinis. Cricetodon brevis from Sansan, being markedly different from the above mentioned, had better be named Cricetodon cf. vindoboniensis SCHAUB & ZAPFE, 1953. It is fairly certain that the species from Villafeliche II A (Spain) is identical to the latter. 4. The group sansaniensis (based on Cricetodon sansaniensis LARTET, 1851) comprises large species. The molars have high relief and fat cusps. Only one locality (Manchones) provided sufficient material for analysis. Comparison of this material with finds from France and North-Spain led to the conclusion that there are two identical lines of evolution within the group. These are most probably closely related, though the common ancestor is not yet known. One branch leads from C. meini n.sp. (from Vieux-Collonges) through a somewhat more highly developed member (from Manchones) to C. decedens (from La Grive-St. Alban, and the Valles-Penades). The other branch, which is also known from Vieux-Collonges, though not yet described, leads to sansaniensis from Sansan. Cricetodon sansaniensis from La Grive seems somewhat more developed and might well be called C. rhodanicus DEPERET. The highest developed member of this branch is found in the Valles-Penades (not yet described). With the above mentioned data of the evolutionary trends in the Cricetodontinae it was tried to correlate some of the well-known Miocene localities in France with the sites in the Calatayud-Daroca region (fig. 17). There proved to be a good agreement between miocene Cricetodon fauna of Spain and France, whereas no similarity was found with the North-African fauna from Beni-Mellal (Morocco). Comparison of the living species Cricetus cricetus canescens NEHRING and Mesocricetus auratus (WATERHOUSE) with the Cricetodontinae of Western Europe shows that none of these can be an ancestor of the recent Cricetinae. Certain primitive characters of the dentition of Cricetus have already disappeared in all known Cricetodontinae. The oligocene genus Cricetops from Mongolia is the only one that more or less satisfies the demands one should theoretically make on an ancestor of the Cricetinae. It is probable that the Cricetinae are originally an Asiatic family, arriving in Europe more or less at the same time as Hipparion and crowding out the Cricetodontinae. The common ancestor of Cricetodontinae and Cricetinae is at least of Oligocene age.