A turbidimetric technique was used to measure the bacteriolytic activities of digestive fluids collected seasonally from the deposit-feeding polychaete Arenicola marina. Lysis of cultured sedimentary bacteria was monitored spectrophotometrically. The kinetics of the lytic reaction were characterized with respect to temperature, pH, reactant concentration, and time. Bacteriolysis generally followed saturation kinetics behavior and was apparently a first-order process with respect to the substrate (bacteria). Deviation from a simple Michaelis-Menten relationship, however, was observed at high substrate levels, at which inhibition of lysis was evident. The lytic rate decreased rapidly and sharply through time as the substrate was depleted. Lytic factors in the gut fluid acted as catalysts; rates increased with concentration of gut fluid, but no depletion in lytic capacity through time was observed. The lytic rate increased exponentially for all seasonal samples over the range of environmental temperatures (4 to 31(deg)C) but showed little relationship with pH under conditions likely to be experienced in situ. The lytic rate also varied among bacterial strains and with culture age.