Enzymes from whole cell acetone powder extracts of 32 axenic isolates of Protosiphon were separated by horizontal starch-gel electrophoresis and assayed for malate dehydrogenase (MDH), alpha-esterases (αEST), and leucine aminopeptidase (LAP). All isolates exhibited MDH and α-EST activity. Only four isolates displayed LAP reaction. The banding patterns were less complex than those of other selected green algae. Enzyme distribution patterns of some Protosiphon isolates correlated with morphological, physiological, and immunological characteristics. In other isolates there was no correlation. Electrophoretic analysis of enzymes appears to be a useful tool in algal systematics.