Specimens of dinoflagellate cysts referable to the cyst-species Spiniferites ramosus (Ehrenberg 1838) Mantell 1854 and Nematosphaeropsis labyrinthus (Ostenfeld 1903) Reid 1974 have been produced in the same crossed-strain of Gonyaulax spinifera-type (Claparède and Lachmann 1857) Diesing 1866 motile cells during a routine encystment experiment. Detailed observations were made and are reported on the process of encystment, which follows a similar pattern to that already described for Lingulodinium polyedrum. Both cyst types displayed a wide range of morphological variations with respect to process length and/or trabecular development. The cysts morphology vary between “typical” S. ramosus specimens with well developed trifurcate processes with bifurcated distal ends, to specimens with abnormally developed or incomplete processes, specimens with partially developed trabecular network, to typical N. labyrinthus specimens with a complete network of trabecula joining the distal ends of the processes. The cysts were produced in salinities of 25 and 30 psu at 12 °C. Experiments in other salinities (15, 20, 35 and 40 psu) only produced cysts of S. ramosus-type. We suggest that the particular salinity conditions (25 and 30 psu) used in our experience probably triggered the expression of a particular phenotype.