The present report is based on the very extensive collections of Hydroidea from the Caribbean region which have been assembled by Dr. P. WAGENAAR HUMMELINCK during his scientific explorations of the fauna of that region. The study of the Sertulariidae, Plumulariidae and Aglaopheniidae, the results of which are contained in the present report, was carried out in the Rijksmuseum van Natuurlijke Historie, Leiden, the Netherlands, from October 1960 till February 1962. Part of HUMMELINCK'S hydroid material has already been studied and reported upon by LELOUP (1935). This material is in the collections of the Rijksmuseum van Natuurlijke Historie, except for some specimens from a small number of localities which are now in the collections of the Institut Royal des Sciences Naturelles de Belgique, Brussels. The material examined by LELOUP – in so far as it is still in the collections of the Rijksmuseum van Natuurlijke Historie and contains specimens of the three families studied – has been included in this survey, but it has only been discussed when the author reached a different conclusion. The present records must therefore be considered as supplementing those given in LELOUP'S (1935) paper; together they illustrate the occurrence and distribution of the various species in the area visited by Dr. WAGENAAR HUMMELINCK [Table 1],