Levels of components of the cytochrome P450 (CYP)-dependent monooxygenase system were characterised in microsomes of major biotransformation tissues, or whole bodies, of 33 species from six phyla of aquatic invertebrates. The phylogenetic distribution of benzo[a]pyrene hydroxylase (BPH) activity in the absence of added NADPH (so-called ‘NADPH-independent BPH activity’) and presence of NADPH was also examined. Microsomal protein yield was higher in individual tissues than whole tissues. The main components (total CYP and cytochrome b₅; NADPH-dependent cytochrome c (CYP) reductase, NADH-dependent cytochrome c reductase and NADH-dependent ferricyanide (b₅) reductase activities) were found in most species of the Porifera, Cnidaria, Mollusca, Polychaeta, Crustacea and Echinodermata examined. The so-called ‘418-peak’ of the carbon-monoxide difference spectrum of reduced microsomes was found in all species, indicating the wide distribution of this protein. Total CYP levels (pmol mg^− 1 protein; mean ± SEM) were similar in molluscs (50 ± 7), crustaceans (61 ± 11) and echinoderms (56 ± 9), with the exception of high levels (223–266) in two crustacean species. NADPH-dependent BPH activity (pmol min^− 1 mg^− 1 protein) was found in 32 species, being lowest in Porifera and Cnidaria (3–4), intermediate in Mollusca (7.8 ± 1.3), and highest in Crustacea (25 ± 4) and Echinodermata (15 ± 4). NADPH-independent BPH activity was evident in 13 out of 15 molluscan species examined, with the addition of NADPH either stimulating (8 species) or inhibiting (5 species) the activity. NADPH-independent BPH activity was also seen in two poriferan species and indicated in three crustacean species, suggesting that the phenomenon is not solely restricted to the Mollusca.