In 8 of 10 enzyme systems, isozymes for Phyllospadix serrulatus Rupr. ex Aschers. from Washington and Oregon were different from those of P. scouleri Hook, and P. torreyi S. Wats, from Washington, Oregon, and California. Plants of P. serrulatus continued to produce the widest leaves in laboratory cultures. Plants of P. scouleri and P. torreyi that differed in leaf width, leaf-tip characteristics, and (or) inflorescences at each collection site continued to show morphological differences in culture but had similar isozyme features in 8 of 10 systems and only slight differences in isozymes of peroxidase and alcohol dehydrogenase. Plants with single-spathe inflorescences, characteristic of P. scouleri, vary widely with broad leaves and rounded leaf tips in the northern part of the distribution and with narrow leaves and notched leaf tips at the southern part of the distribution. Plants with multiple-spathe inflorescences, characteristic of P. torreyi, usually have narrow leaves with notched leaf tips throughout the distribution on the West Coast. Some plants from the Monterey Peninsula continued to produce wide leaves, notched leaf tips, and multiple-spathe inflorescences in culture but had isozyme patterns that were more similar to those of P. scouleri, suggesting possible hybridization in the southern part of the range.