After 2 years of big catches (over 100 000 tonnes annually) of large (up to 85 cm ML) jumbo squid (Dosidicus gigas) during La Niña years, the fishery vanished in the Gulf of California during 1998 following the 1997-1998 El Niño. This event impacted jumbo squid abundance and population structure in the region. In 1999, abundant medium size maturing squid (<45 cm ML) were found in several locations in the Gulf of California, where 58 000 tonnes were fished. The environment during the transition phase to the 1998-2001 La Niña conditions may have favoured the presence of a medium size, early maturing form of jumbo squid as opposed to the large size squid fished during previous La Niña years. The return of large size maturing squid to the Gulf in 2000 may have occurred through a gradual increment of size at maturity. Additional information is reported on sex ratios, copulation rates, and gonadosomatic indexes based on 732 individuals of this medium size maturing jumbo squid from the Gulf of California and adjacent waters. Inglés