During an oceanographic cruise off Santa Catarina (Lat. 26°S to 27°S) in November/1994 we have observed conspicuous neustonic blooms of the cyanobacteria Trichodesmium hildebrandtii. Two features were distinguished in the blooms: patches composed by brownish healthy colonies (5 colonies/L) and patches composed by whiteness senescent colonies (1200 colonies/L). Quantification of the phytoplankton and protozooplankton associated to the patches revealed densities up to 7 times higher in samples from the senescent bloom than those from the healthy one. According to the results, we suggest that the degree of senescence and decomposition of the cyanobacterial blooms can determines changes in the structure of the associated planktonic assemblages. The bloom as a whole seems to be seasonal and related to the increased influence of tropical water massas in spring and summer months.