Records of selected gymnodinioid dinoflagellates from the open waters in the vicinity of the Kuroshio and Oyashio Currents, the Philippine, Celebes, Sulu and South China Seas, western and central equatorial and southeast Pacific Ocean are described and illustrated. The species Gymnodinium fusus Sch?tt, Gyrodinium falcatum Kofoid et Swezy, G. caudatum Kofoid et Swezy, G. sugashimanii J. Cachon et al. and Pseliodinium vaubanii Sournia are considered to be morphotypes of a single species, that until further studies can establish the correct genus, are named G. falcatum. This study is the first to record individuals of G. falcatum with very long curly extensions. Other gymnodinioid dinoflagellates that showed bifurcated hyposomes may be related to Gyrodinium bifurcatum Kofoid et Swezy or cells of thecate dinoflagellates exuviated from their thecae. Some specimens showed a rigid cover, although no discernible thecal plates. In this group, the most common species was Ptychodiscus noctiluca Stein and, for the first time, a micrograph of a tentative specimen of the genus Berghiella Kofoid et Michener is reported. The validity of the genera Berghiella and Balechina Loeblich Jr. et Loeblich III with thick cell covers is discussed. Several species with apical extensions, other unknown taxa with distinctive shapes, and colonial forms are illustrated. The diversity of gymnodinioid dinoflagellates is underinvestigated in the open ocean.