A feral pig population on Santa Catalina Island, California, was studied for 17 months beginning in July 1980. A total of 136 pigs was marked and 188 recaptured during capture-recapture studies at two sites. Density was estimated to be 21 to 34 pigs/km2 (95% confidence interval). Dry season home ranges determined from radio-telemetry data were small and differed significantly between boars (2.04 ± 0.20 km2—95% ellipse; 2.44 ± 0.30 km2— harmonic mean method) and sows (0.90 ± 0.23 km2—95% ellipse; 1.46 ± 0.14 km2—harmonic mean method). During the dry season, pigs preferred cool moist canyon bottoms, the result of both a physiological need for free water and behavioral responses to high environmental temperatures. Patterns of utilization during the wet season appeared to be primarily a function of food availability. Seasonality in time of birth was evident, with piglets born in winter and spring (1980) or spring and summer (1981). Litter size as measured by fetal counts was 5.00 ± 0.36 and sows averaged 0.86 ± 0.17 litters over a 12-month period. Most sows were older than 1 year when they first conceived, and litter size increased from puberty and peaked at 2–3 years of age. Intrauterine mortality of fetuses averaged 25% and piglet mortality was estimated to be 58% prior to weaning. Mortality rates appeared to be higher for piglets born in summer than in winter or spring.