The phosphomonoesterase activity of a microbialcommunity dominated by Chroococcidiopsis in arock sample from the Antarctic dry desert was comparedwith that of a non-axenic culture of thecyanobacterium isolated from the same rock in order toestablish whether the organism differed in itsproperties when immobilized in the rock. The pHoptimum (9.5) was the same, but the response totemperature and light both differed. In the case oftemperature, the optimum was much higher in culture. Light flux at the low value of 8 mol photon m-2 s-1 was slightly inhibitory for acrushed rock sample, but enhanced activity of thecultured material. Although materials both showeda typical (apparent) Michaelis-Menten relationship,the values for Km and Vmax differed. Itis suggested that study of natural immobilized systemsis important for the development of procedures to useimmobilized phototrophs for practical purposes.