The tropical sea urchin Eucidaris tribuloides (Lamarck) was placed under two variable and two fixed photoperiod regimes at a constant temperature of 22 °C for a period of 1 yr. Gametogenic condition was ascertained at time zero (June 1988) and then every 3 months thereafter by scoring the ripeness of the gonads (presence of gametes oozing from gonad, microscopic examination of egg sizes and sperm activity from smears), measuring the gonadal index, and histologically analyzing subsamples of the ovaries and testes. Individuals held under ambient in-phase photoperiod regimes (14L : 10D summer; 10L : 14D winter) developed mature gonads in September 1988 in synchrony with the known annual reproductive cycle of field populations. Individuals held under photoperiod regimes 6 months out-of-phase with ambient photoperiod did not become reproductively mature until March 1989, indicating that gametogenesis had been delayed until the days shortened or nights lengthened during the artificial "winter" photoperiod. Sea urchins held under fixed long days (15L : 9D) delayed gametogenesis over the entire year, developing mature gonads in ≈ 20% of the individuals. Sea urchins held under fixed short days (9L: 15D) were stimulated to produce mature gametes throughout the year, and mature gonads were found in ≈ 60% of the individuals. These observations suggest that short days or long nights enhance and entrain gametic development in E. tribuloides. This is the first documentation of photoperiodic control of gametogenesis in a tropical echinoderm, and indicates that marine invertebrates may be able to cue reproductive events using relatively moderate, but highly predictable, incremental changes in annual photoperiod. © 1990.