Ontogenetic changes in the body morphology, morphometrics, age, growth, maturation, fecundity, feeding spectrum and parasites were studied in 218 specimens of the oceanic nektonic squid Onychoteuthis banksi and in two specimens of the rare planktonic squid Chaunoteuthis mollis (Onychoteuthidae) caught in the tropical waters between 1977 and 1990. Similarities between the two species in major taxonomic features and in several morphometrical indices suggest that C. mollis is in fact a mature female of O. banksi with degenerated mantle and fin muscles and lost tentacles. Thus, C. mollis will be considered as a lesser synonym of O. banksi. Statolith microstructure analysis reveals that O. banksi is a slow-growing squid with low maturation rates, spending the first 240–260 d of life in the epipelagic zone. The main features of the reproductive strategy include earlier maturation of the males, small eggs (0.2–0.5 mm in diameter by the end of protoplasmic growth), synchronous development of oocytes and high potential fecundity (51,000–205,000 eggs). Onychoteuthis banksi is an opportunistic predator, shifting to a fish and squid diet much earlier and at smaller sizes (20–40 mm ML) than ommastrephid squid. The feeding activity of small and medium O. banksi (20–80 mm ML) is a little higher and that of large O. banksi (80–148 mm ML) is lower than in ommastrephids of the same size. The helminth fauna of O. banksi is impoverished and consists of ‘food parasites’: larval Didymozoidea and Scolex spp. The level of infection is low, only reaching -52 specimens per host in large squid. The nature of the life cycle of O. banksi and possible evolutionary trends in the development of the ‘Chaunoteuthis’ stage for maturation and spawning of females in the deep water are discussed.