A study was carried out on the relationship between hair formation and water chemistry in three genera of Chaetophorales (especially Stigeoclonium).The relationship between environmental chemistry, algal morphology and phosphatase activity for 32 samples of natural populations in N-E. England was investigated. Two indices of hair formation were significantly correlated with aqueous P (-ve).algal N: P ratio (+ve) and phosphatase activity (+ve). Other correlations included: algal P v. aqueous P {+ve); algal P V. phosphatase activity (-ve); algal N: P ratio v. aqueous P (-ve). All algal samples with a P content of 0.96% (by weight) and above lacked hairs: all with aP content of 0.53% and below had hairs. All algal samples with N: P ratios of 5.58 and below lacked hairs: all with ratios of 6.46 and above had hairs. The influence of pH on phosphatase activity of six natural populations showed three markedly different responses, but the pH optima of three laboratory strains of Stigeoclonium were similar; pH had little influence on the phosphatase activity of two laboratory strains of Draparnaldia. Phosphatase activity was localized on hairs of Chaetophora and Draparnaldia, but the results were not clear-cut for Stigeoclonium. Elemental deficiencies gave a similar pattern of the extent of hair formation in 13 strains: - P > - N >- Fe-S >-Ca > -Mg > control. Elemental additions to five strains resulted in zoospore release; the greatest amount was released when N or P was added to N- or P-limited cultures. Five strains were grown to P-limitation in batch culture: the maximum P composition ranged from 2.4 - 6.4 % dry weight; hair formation commenced when algal P had fallen to about 1 % dry weight, long before any culture had ceased to grow or started to appear unhealthy. The strains were shown to be capable of utilizing a number of organic P sources. The possible role of hairs and similarities between hair-forming blue-green algae and Chaetophorales are discussed.