The population biology of the tropical marine gastropod Conus pennaceus was studied off the island of Oahu in the Hawaiian Islands. Growth, fecundity, and survivorship rates were determined in the field through a combination of mark—recapture studies, regular population censuses, and direct count of ova produced by individual females. Individuals of C. pennaceus grow to a shell length of 15 mm in their 1st yr, and females may reproduce by the end of their 2nd yr at a shell length of between 20 and 25 mm. Females probably produce 1 egg mass/yr, and age—specific fecundity increases with increasing female size. By year 10, annual fecundity of females has increased to over 6000 ova, while shell growth has slowed to<1mm/yr. Although C. pennaceus has no planktonic larval stage, over 99.93% of all fertilized ova fail to survive their 1st yr. Subsequent mortality is constant at an annual rate of 0.419, and very few animals survive to the end of their 10th yr. The population examined remained relatively constant in size during the 22—mo study. This stability is attributed to the shape of the reproductive value curve of the population and to the ability of C. pennaceus to utilize a wide variety of prey species.