1. Fruiting margins of Ulva lactuca thallus are characterized by a higher content of carotenoids, as well as by higher respiratory and lower photosynthetic activity, than the corresponding vegetative thallus. Photosynthesis in the male margin was also characterized by a longer induction period than was observed in the vegetative thallus by the polarographic method. 2. The changes in pigment composition observed in Ulva lactuca during reproduction are similar to those reported for Ulva lobata. The male gametes of Ulva lactuca contained twice as much carotene and half as much chlorophyll as the female gametes. The male carotene fraction contained 43% gamma-carotene, 32% beta-carotene, 3% alpha-carotene and 22% unidentified carotene. The xanthophyll contents of the male and female gametes were similar. The chlorophyll/carotenoid ratio was 8.3 in the vegetative thallus, 2.7 in the female gametes and 1.1 in the male gametes. 3. The respiration of the male gametes exceeded that of the female gametes, was almost thirty times greater than that of the vegetative thallus on a solids basis, and was about twelve times greater in terms of chlorophyll content. 4. Both male and female gametes exhibited net photosynthesis in intense white light, but a very high light intensity was required to compensate their respiration, which is the highest that has been reported for cells possessing photosynthetic activity. 5. Positive phototaxis of Ulva gametes is a response to blue-violet light, which orients their swimming but does not otherwise control their flagellar activity. Their maximum rate of linear movement is attained at moderate light intensity (ca. 100 f.c.). The gametes swim 2500 times their own body length each minute.