Adult Geitoneura klugii klugii (Guerin-Meneville) and G. acantha ocrea (Guest) were collected throughout the 1985-86 flight season. Females were dissected to determine mating state and ovarian development on each occasion. Fecundity and duration of egg development were measured. Most females mate when pre-reproductive. Females were predominantly monogamous, and few instances of multiple insemination were revealed by spermatophore counts. Both sexes are reproductively active soon after emergence and neither species experiences any major delay in ovarian development. Oviposition occurs over an extended period with most eggs deposited singly. Fecundity of the two species is similar: follicle counts gave estimates of 200-280, and females laid similar numbers of eggs, up to 399 (G. klugii) or 323 (G. acantha). Eggs of G. acantha hatched, at a range of temperatures, within 2-5 weeks. In contrast, eggs of G. klugii developed but did not hatch, and it is likely that this species has a pre-hatching diapause. We suggest that this major difference between the two closely related co-existing species may reflect adaptations to preferred larval food-plants.