In 1967 we examined the crayfish collections in the Rijksmuseum van Natuurlijke Historie, Leiden, to determine if the crayfishes collected in New Guinea in 1954 by Dr. L. B. Holthuis, Dr. L. D. Brongersma, and Dr. M. Boeseman (Boeseman, 1963) were hosts to commensal ostracods. Of the collections examined, only the first contained ostracods. Forty-nine specimens were obtained from that collection, and are here described as a new genus and species belonging to the subfamily Notocytherinae. Elachistocythere gen. nov. Diagnosis. — A genus of the entocytherid subfamily Notocytherinae. Antennule with six podomeres. Dorsal antennal claw subspiculiform and devoid of setae. Mandibular protopodite with distal row of five teeth. Peniferum bifid. Penis straight, ventrally directed, and situated slightly below mid-length of peniferum. Commensal on freshwater crayfishes in New Guinea. Type-species. — Elachistocythere merista sp. nov. Name. — From the Greek elachistos = small, and cythere. Elachistocythere merista sp. nov. Male. — Shell (fig. 1) subelliptical in outline with eyespot situated approximately 1/4 shell length from anterior end. Measurements of 49 specimens are given below in table 1. TABLE ι Length and height ranges and averages for two immature stages and adults of Elachistocythere merista sp. nov. Immature Immature Adult Immature Adult (4 legs) females females males males