Heterotrophic protista have been shown to be important components in aquatic food webs. The goal of this research program is to develop a new methodology for studying protistan ecology in aquatic environments. Present methodologies for examining these assemblages cannot resolve the trophic status of some important species, nor do they facilitate studies of their spatial and temporal variability. The identification of many of these species in natural samples is presently dependent on procedures that are too laborious (e.g. electron microscopy) or unrealistic (e.g. the observation of live specimens) for routine use. This project will develop a new methodology, based on the sequencing of ribosomal RNA genes, to construct fluorescently-labeled oligonucleotide probes for the identification of protists in mixed assemblages. Moreover, it will establish a framework by which other investigators may adapt the methods for the development of probes in their own work.