Deoxyribonuclease activity (DNase) was studied in crude extracts prepared from the digestive tracts of the marine molluscs Aplysia dactylomela Rang, Haliotis coccinea canariensis Nordsieck, Littorina striata King et Broderip, the echinoderm Diadema antillarum Phillipi, and a commercially available abalone acetone powder (AAP; Sigma, Ref. A-7514) to evaluate their toxic effects when used for seaweed protoplast isolation. DNase was detected at pH 5.0, in presence of Mg²⁺, in all the crude extracts except in those obtained from the digestive gland of A. dactylomela. At pH 6.0, activity was not detected. Maximal specific activities were measured in L. striata and D, antillarum crude extracts. AAP showed the lowest specific activity of all the assayed extracts. The pH in the different digestive organs of these phycophages ranged from 5.0 to 7.0, measured in vivo, providing new information about the optimum values at which digestion occurs and, together with results on DNase activities, those organs which should be used to degrade seaweed cell walls.