A study was carried out on phosphatase activity, phosphate uptake and its relationship to hair formation in the Rivulariaceae. The Rivulariaceae was chosen as it is a widespread taxon, where hair formation is a common occurrence, and previous studies indicated that they originate from environments where a large proportion of the phosphorus (P) is present as organic P. It seems possible that hair-forming Rivulariaceae are especially well adapted to utilize organic P. Initially 51 axenic cyanobacterial strains, from 10 genera, were screened for yields using organic P sources and for cell-bound and extracellular phosphomonoesterase (PMEase) and phosphodiesterase (PDEase) activities. All strains exhibited detectable inducible PMEase activities, and highest cell-bound PMEase activities were in hair-forming Rivulariaceae. Synechococcus had significantly low cell-bound phosphatase activities and five strains were unable to hydrolyze phytic acid. PDEase activities were lower compared to PMEase activities in all strains. Strains isolated from deepwater rice habitats had significantly higher levels of PDEase activity. In the three Calothrix strains tested, Calothrix 202, 550 and 603, inducible phosphatase activities were similar whether the P source was inorganic or organic. PMEase synthesis in these strains began when cellular P (% dry wt) values were in the range 0.60 - 1.0%. Differences in the influence of environmental variables on cell-bound and extracellular PMEase activities in hair-forming Calothrix 550 were slight, suggesting that PMEases in the two fractions had a common origin. Of the eleven ions tested Ca had the most pronounced stimulatory effect on PMEase activity. Localization of enzyme activity in Calothrix 550 suggested that the enzyme was bound to a surface. Partial purification of an extracellular PMEase fraction detected four bands of PMEase activity on a non-denaturing polyacrylamide gel. Three of the four bands were associated with carbohydrate and the bands were not extractable by mechanical means. Localization of PMEase activity in hair-forming strains by azo dye (naphthol AS-MX) and 5-bromo-4-chloro-3-indolyl phosphate (BCIP) showed that PMEase activity was associated with hair cells. Phosphate uptake experiments with Calothrix 253 and 550 suggested that uptake at high external phosphate concentrations was located in hair cells. NaCl, above 67.5 mM, inhibited hair formation and subsequently phosphatase activity in Calothrix 253 and 690. Addition of mannitol or sorbitol had no effect on hair formation, suggesting inhibition of hair formation was not an osmotic effect. Removal of P-deficient cultures from saline to freshwater media led to a marked synchronization of hair formation (in 90% of trichomes) and increase in cell-bound PMEase activity. Localization of cell-bound PMEase activity by light microscopy, using naphthol AS-MX, detected activity in the hair cells.