This project focused on the challenge of obtaining near–real-time data on the abundance (spatial and temporal) and toxicity of algal cells for the routine monitoring and scientific investigation of harmful algal blooms and their associated toxins. The primary impediment to obtaining such information is a lack of appropriate, field-tested methods. This project was designed to resolve this problem by evaluating the performance of two techniques that lend themselves to rapid detection of algal cells and their toxins: species-specific DNA probes and phycotoxin receptor binding assays, respectively. The project focused on populations of domoic acid (DA)-producing diatoms of the genus Pseudo-nitzschia that occur in Monterey Bay, California. Species-specific probes targeting Pseudo-nitzschia were applied using both whole-cell and sandwich hybridization methods, with good agreement between methods observed during the early stages of bloom development when cells were actively growing. Although some discrepancies arose toward the latter phase of a bloom, due largely to underestimates using the whole-cell technique with cells in poor condition, both approaches were generally effective in detecting toxic cells at concentrations relevant for routine monitoring. The receptor binding assay reliably detected DA associated with cells during periods of bloom development, but the assay occasionally revealed DA in the absence of recognizable, intact cells as blooms declined. Precise estimates of cell abundance and toxicity may not be required for routine monitoring. Perhaps most important for public health officials and wildlife biologists are up-to-date trends of where potentially toxic Pseudo-nitzshia species are; whether the populations are on the rise, falling, or exceeding some thresold value; and whether any associated toxicity warrants attention. In this regard, sandwich hybridization application of DNA probes for Pseudo-nitzschia and the DA receptor binding assay are technologies that show good promise and should be tested further against currently used methods in collaborative trials.