Rotifers (Brachionus spp. ) are one of the most important live prey items used in aquaculture, and over the past few years, they have been increasingly utilized to feed larval zebrafish (Danio rerio). One of the most widely employed rearing protocols now used in this species is the "polyculture" approach in which larval fish and rotifers ( B. plicatilis type L) are grown together in static, brackish water for the first 5 days after gas bladder inflation. We have recently observed that mortality events occur approximately 12-18 hours after fish are in itially introduced to live rotifers. We attempted to understand the physiological mechanism underlying fish mortality by determining the optimal developmental stage (4, 5, 6, 7, 8, or 9 dpf ) to inoculate larval zebrafish with rotifers, and if rotifer density (low vs. high) affects survival. W e predicted that mortality rate would be positively correlated with later inoculation (6 dpf onwards) and high rotifer abundance. Survival was twice-daily monitored over 14 days of larval zebrafish inoculated in high (100/ml) or low (10/ml) rotifer densities between 4-9 dpf . Histopathological examination of affected fish showed distension of the intestine and severely abraded and vacuolated epithelial cells , which suggests leeching of gut bacteria to the coelomic cavity. To avoid this phenomenon, this study suggests that early inoculation (4 or 5 dpf ) of larvae pro motes the most optimal survival. In addition, survival is higher in polycultures with low rotifer density.