Two marine bacterial isolates differ considerably in their uptake rates of extracellular products produced by the alga Skeletonema costatum: 0.4 × 10−8µg C cell−1 hr−1 for spirillum 7697; 16 × 10−8µg C cell−1 hr−1 for pseudomonad HNY. These disparate uptake rates are used to interpret the growth patterns of the bacteria in the presence of the alga on a seawater inorganic enrichment medium. Pseudomonad HNY grows well with the alga in both batch and continuous culture, in the latter attaining a steady state population two orders of magnitude greater than achieved in the absence of the alga. Spirillum 7697, however, grows poorly in the presence of growing algal cells and appears to be inhibited to some extent under such conditions. Such observations suggest that dominant bacterial populations associated with algal blooms are a result of both stimulation and inhibition mediated by the release of extracellular products.