Measurements were made on 274 spawn clumps and 271 adult frogs in three consecutive years at breeding sites in the west of Ireland. Mean clutch size was 1008 ± 20 ova, mean egg diameter was 2·32 ± 0·01 mm, and mean clutch weight was 11·62 ± 0·37 g. At any particular body length, larger clutch sizes were associated with smaller egg sizes and vice versa. Clutch size, egg size and clutch weight all increased allometrically with body size. The bigger females produced more and smaller eggs and a heavier clutch, relative to their body size, than smaller ones. Egg size and clutch weight also increased allometrically with female age. Age-specific clutch weight increased during the first two years of maturity and declined slightly thereafter. Age-specific egg size increased with age throughout life. At any age, bigger females produced more but not larger ova, and at any length, older females contained larger but fewer eggs. Both bigger and/or older females exhibited greater variability in the reproductive parameters than smaller and/or younger ones.

Laboratory mating experiments showed that egg fertility was positively correlated with absolute male length and with the relative male/female length in amplexus. The fertilizing ability of males, however, declined with mating frequency, irrespective of male length, clutch size or the time interval between successive matings. In the wild, egg fertility ranged from 24–100%, although over 90% fertility was common.