The heterogeneity of the peroxidases in peas was examined by starch gel electrophoresis. Comparisons were made between tall and dwarf cultivars and among organ systems developed in light and darkness. Isoperoxidase bands could be grouped as cathodic, anodic and near-neutral (at pH 9.0) types. The cathodic set stained well with guaiacol oxidation products whereas some anodic bands reacted preferentially with 2,6-dimethoxyphenol. Some near-neutral bands were aceto-carmine positive and may have been organellar. Each organ had a characteristic isozyme pattern, and the band patterns in corresponding organs from different varieties were far more alike than were the patterns in the different organs within each variety. Ontogenetic changes were marked in all 3 organ systems, principally in the cathodic bands. The effect of light on isozymal patterns was quantitative rather than qualitative, possibly influencing the isoperoxidases secondarily via its effect upon organ physiology and development.