Age-0 largemouth bass Micropterus salmoides were collected over seven years throughout the spawning season in Lucchetti Reservoir, Puerto Rico. Sagittal otoliths were removed and daily ages determined to assess hatching periodicity. Hatch date distributions indicated that the major spawning of largemouth bass started in January, soon after photoperiod began to increase. The time lag between increasing day length and the start of spawning, however, was dictated by water level fluctuation. In some years, when the water level began to decrease during the spawning season, spawning was interrupted, but resumed when water levels started to rise again, resulting in bimodal hatch-date distributions. Because of its tropical geographic location, Lucchetti Reservoir experiences a temperature range of 24-30degreesC annually, and relatively constant temperature during the initiation of spawning. Our data indicate that, despite some role of photoperiod in defining the primary spawning season, the initiation of largemouth bass spawning was stimulated by water level increase in this tropical reservoir. The overriding role of water level was further evident by late-season pulses in spawning in some years, that were related to rising water level. Water level increase during the spawning season is a potential management tool for stimulating largemouth bass spawning in systems where water temperature is suitable.