A study of the reproductive biology of the loliginid squid, Alloteuthis subulata in the North Sea, Irish Sea and Portuguese waters was carried out. A predominance of small squid (<50 mm ML) during autumn was observed in all three areas. Multi-modal size-frequency distributions were apparent in both sexes. The greatest complexity was observed in Portuguese waters, with males exhibiting possibly four or more modal size classes. Sexually mature squid were found throughout the year, with generally more mature squid in spring-summer and fewer in autumn-winter. In Portuguese waters, maturity in both sexes peaked in spring, whereas maturity in the other areas peaked in summer. Similar sizes-at-maturity were observed, with male squid maturing over a greater size range (40-125 mm ML) than females (30-50 mm ML). An apparent secondary peak in male maturity at 65 mm ML suggests two different size-maturation patterns in Portuguese waters. Estimates of potential fecundity in the Irish Sea ranged from similar to 2200-13 500 eggs per female. Mean egg diameters of 0.45 mm and 1.55 mm were recorded for developing oocytes and mature ova, respectively. Three to four modal size classes of oocytes were apparent in each ovary. There was some indication of latitudinal effects on growth, maturation and reproduction of A. subulata across its geographic range. The greater complexity of size-structure in warmer waters suggests that water temperature may be the main factor involved, possibly through shorter incubation times, faster growth and maturation rates and extended spawning periods.