Lago Bonilla, located 370 m above sea level on the Eastem slopes of Volean Turrialba, Costa Rica, was visited four times during 1990-1991. Basic Limonological data were recorded in each visit. lt is oligomietic, mixing during the coldest and windiest months sometime between Nov. and Feb, and eutrophic, with low Secchi transparencies, Cyanobacteria dominance and almost permanent anoxic conditions in the deepest portion. A shift in N to P ratio was observed, indicating a variation in the limiting factor during the year; N beco mes Iimiting during the driest months probably as a result of a lower input from the catchment due to lower runoff, coupled with a higher P input from the sediments during deep mixing periods. H owever, Iimitation by P was probably stronger than N when it occurred, according lo the N/P ralios. Phytoplankton in epilimnetic waters comprised 58 algal species, and was dominated by Cyanobacteria and Chlorophyta. Only a few species dominated Ihe community, mainly Cylindrospermum mínímum. and Tetraedron mínimum, rendering low diversity values. Zooplankton was also poor: only seven species were found. There were some indications of vertical migrations of sorne zooplankters