The short-term dynamics of virus-like particles (VLPs) abundance, bacterioplankton, ciliates and flagellates were analyzed in a small tropical lake, during a rainy day (June 9-10, 2003) and a dry day (February 18-19, 2004), with intervals of 3h between the samplings. Frequent sampling in intervals of 15min were conducted. During the rainy day, the VLP mean abundance was 7.0x108mL(-1) and bacterial density was 5.75 x 107 mL(-1). During the dry day, VLP and bacterial mean were 5.78 x 108 and 4.1 x 107 mL(-1), respectively. The virus/bacterium rate (VBR) varied from 11 to 18 on the rainy day and from 4 to 22 on the dry day. The density of VLP was higher during the night, especially on the dry day, suggesting a virucidal action of the solar radiation on them. When registered in intervals of 15min, the densities were not associated with the fluctuations of bacteria or chlorophyll a (Chl a), but a strong negative correlation between VLP and protozooplankton was observed (Spearman: R=-0.71; p=0.04), possibly associated with the occurrence of viral lyses on these organisms. The variations of VBR in the system, indicate that the elevated densities and fluctuations of VLP is suggestive of an active and important participation of these biological entities in the dynamics of the microbial communities in the studied environment.