Phytoplankton of the Botos lagoon of Poás volcano is dominated by dinoflagellates. The two species of greatest abundance in March 1979 were Peridinium incospicuum Lemm. and P. volzii Lemm, which represented 96% of total cells. The identity of these species was confirmed by electron microscopy. Both species are cosmopolitan. Without further resealch it is premature to compare Botos Lagoon with similar lakes in Central America.