(1) According to Weismann, in the life-cycle of Moina sexual forms should be produced in every parthenogenetic generation independently of external conditions. (2) By isolating the parthenogenetic females at birth until the production of the brood at a temperature of 25° to 30° C. the production of sexual forms is entirely suppressed, 1167 parthenogenetic females and no sexual forms having been obtained by this means. (3) Parallel cultures with related females crowded together in the culture-glasses at a temperature of 25° to 30° C. gave 30.3 per cent. males, and at about 14° C. gave 52.3 per cent, males. Isolated females at 14° C. gave 19.1 per cent, males. Isolated females in an ice chest at about 5°C. gave 5.4 per cent. males, when crowded together 42.5 per cent. (4) The intensity of crowding, measured by the number of parents kept together in the same glass, is shown to have a not very constant effect on the proportions of males produced, on the whole the proportion of males increasing with the intensity of crowding. (5) The influence of isolation and of a high temperature on the suppression of the sexual forms may be ascribed either to the comparative absence of excretory matter under these conditions, or else to the nutritive conditions being under these circumstances highly favourable.