Newly developed electrochemical methods makes it possible to quantitatively detect low concentrations of DNA or RNA from harmful algal species. We will present the basic theory behind electrochemical detection and describe our initial attempts to design a convenient assay for quantitatively detecting Pfiesteria piscicida and the Pfiesteria- like species Cryptoperidiniopsis brodyi in natural water samples. The electrochemical detection method being developed is a modified sandwich hybridization technique. For this assay to work, species-specific gene sequences that can serve as hybridization targets must be identified. Suitable target sequences have been identified from the ribosomal 18S and ITS regions of P. piscicida and C. brodyi. A short peptide nucleic acid (PNA) probe homologous to the unique target DNA or RNA sequence is then synthesized, labeled with biotin, and attached to a neutravidn-coated electrode. The PNA sequence of the labeled probe then binds the target RNA or DNA from the lysed cells. This forms the first part of the hybridization sandwich or "bridge". At the same time, a second PNA probe labeled with horseradish peroxidase (HRP) also binds to the target DNA or RNA. The binding of the second probe completes the hybridization sandwich. When hydrogen peroxide and an electron transfer mediator are added to the system, a strong catalytic current is produced from the HRP-labeled probes localized by hybrid formation at the electrode surface. The catalytic current is proportional to HRP bound, which in turn is proportional to the amount of target DNA or RNA present. The electrode detection system is highly sensitive, portable, and inexpensive to manufacture.