The South American species with the vernacular name "mollies" are analyzed and three new species of the genus Poecilia are described and figured, viz., P. boesemani n. sp. from Trinidad, P. koperi n. sp. from Venezuela and Colombia, and P. wandae n. sp. from Lake Maracaibo. Thirteen species of Poecilia are presently recognized from the northeastern part of South America and adjacent islands, viz., six mollies (including P. vivipara and P. caucana), five micropoeciliids, P. heterandria and the guppy (P. reticulata). Two different populations in both P. koperi and P. wandae are recognized, based on variation in pigmentation. From the type series of P. dovii Günther, 1866 and P. vandepolli Van Lidth de Jeude, 1887 lectotypes are selected. Four subspecies of P. gillii are defined as morphologically distinct and geographically correlated populations.
Poecilia reticulata, P. heterandria, and the micropoeciliids P. bifurca, P. parae, P. picta, P. branneri and P. minima are not considered, whereas P. laurae remains a species inquirendae. A geocline in characters of the species of Poecilia is recorded and a possible evolution is discussed.