Assimilation and respiration by a marine Pseudomonas sp. was evaluated to determine whether the activity of bacteria attached to solid surfaces (substrata) differed from that of free-living bacteria. Bacteria were allowed to attach to glass and plastic substrata with a range of wettabilities, evaluated by measuring water contact angles. Amino acid assimilation was determined by microautoradiography and liquid scintillation counting, and respiratory activity was determined from 14CO2. evolution and electron transport system (ETS) activity evaluated by tetrazolium staining. The uptake kinetics of leucine demonstrated thatfree-living cells had a smaller half-saturation constant than any of the surface-associated populations but a somewhat greater maximum velocity of uptake than the cells associated with all but the most hydrophobic substratum. Pre-attachment incubation of free-living bacteria with 3H-leucine resulted in the subsequent attachment to all the substrata of approximately a four fold higher percentage of labelled bacteria as compared with populations that remained unattached. When incubated with amino acids at a concentration of l0μg C l-1 after attachment, the proportion of attached bacteria that assimilated amino acids and the rate of assimilation per cell, was greater than, or similar to, that of free-living bacteria. However, the proportion of attached bacteria that demonstrated ETS activity and the rate of 14CO2 respired per surface-associated cell, was less than, or similar to, that of free-living bacteria. There was little relationship between substratum wettability and the activity of attached bacteria at low substrate concentrations (≤ 10μg C l-1) but in the presence of higher leucine concentrations the proportion of attached bacteria that assimilated leucine and demonstrated ETS activity increased with substratum hydrophilicity. The relative activities of attached and free-living bacteria depended upon the substrate, its concentration and the substratum properties. However, the efficiency of substrate utilisation by attached bacteria was generally greater than that for free-living bacteria.