The biochemical and energetic composition, population biology (size-weight relationship, abundance and size-frequency distribution) and chemical defense of the antarctic ascidian Cnemidocarpa verrucosa Lesson was investigated at McMurdo Sound, Antarctica, during the austral summer of 1989. The organic content (% organic material · g-1 dry tissue wt) of individual body components (tunic, body wall, endocarps, intestines, mature ovitestes and branchial basket) was generally high, with values ranging from 44.5% in the endocarps to 83.9% in the gonads. Most of this material was in the form of NaOH-soluble and insoluble protein. Tissue energy values ranged from 15.1 (tunic) to 22.4 (gonad) kJ · g-1 dry wt. Body height (cm) was positively related to body weight (g dry wt) by an exponential function. A representative individual (14 cm height, 550 g wet wt) contained a total of 493 kJ with most (75%) of this energy attributable to the body wall and tunic. The gonadal index [(kJ ovitestes · total kJ-1) x 100] for sexually mature individuals collected in November was 17.2 ± 4.7 (n = 6). Population densities of C. verrucosa at depths of 20-30 m were 0.4 ind · m-2 at a site 3 km north of McMurdo Station. Energetic densities were estimated to be 197 kJ · m-2. Size-frequency analysis revealed four modal peaks that probably represent distinct age cohorts, and may indicate predictable, annual recruitment events. Bioassays revealed that the tunic was noxious to sympatric pelagic and benthic fish, as well as an allopatric model fish. However, aqueous tunic extracts did not cause mortality in sea urchin sperm indicating the noxious compound(s) is not cytotoxic. pH values for body components were weakly acidic or neutral (5.86-6.93). Mature ovitestes were rejected by sympatric pelagic fish suggesting that gametes may be chemically defended. Although this common antarctic ascidian represents a significant resource of materials and energy, its tunic is tough and noxious, and probably provides an effective means of defense against potential predators. © 1991.