Investigations on the sexual maturity, spawning and fecundity of king weakfish, Macrodon ancylodon, off the southern coast of Brazil (lat. 29º to 32ºS) were carried out from September, 1976, to August, 1977. The sex ratio varied monthly, with slight predominance of males (51.14%) over females (48.86%). The mean size at first maturity was estimated as 27.4 cm TL for females and 21.5 cm TL for males. The macroscopic examination of gonads and analysis of the gonadal index indicated that December and February were the most intensive spawning months. Fish weight proved to be the best predictor data of fecundity: F = 17056.2500 + 425.3712 Wt (r² = 0.6434) , were F is the total number of eggs in both ovaries. The number of eggs of 199 females varied from 59,850 for 33.4 cm TL fish to over 358,450 for 37.2 cm TL; the mean fecundity verified was 165,273 eggs per female. Frequency distributions of egg diameter in the ovaries were polymodal, suggesting fractional spawning during the reproduction season.

Macrodon ancylodon was obtained from landings in Santos Port (São Paulo State) of fish caught monthly by commercial vessels, on the southern coast of Brazil during September 1976 to August 1977. The measurements of total lenght, total weight determination of sex and maturity stages were made. The food items found in stomach contents were: shrimp (Artemesia longinaris), fishes (Paralonchurus brasiliensis, Macrodon ancylodon, Trichirus lepturus, Gadidae, Engraulidae), molluscs (Loliginidae, Bivalvia) and Stomatopoda. Widening of food spectrum was observed in winter. The index of preponderance showed that immature females, immature and mature males fed predominantly on shrimp, while mature females fed mainly on fishes. During spring and summer (spawning season) M. ancylodon exhibited relatively low percentage of full stomachs. Cannibalism was encountered more intensively for mature fish.

Multiple range tests according to Kramer, 1956 were performed on 3 meristic and 15 morphometric characters of "pescada-foguete" (Macrodon ancylodon) samples collected off coast near Conceição da Barra (Espirito Santo State), Atafona, Macaé, Ubatuba cities, Bom Abrigo Island and Rio Grande do Sul State coast. It was also observed morphological differences in otoliths collected at each region. The number of statistical differences among samples and different morphological characteristics presented by otoliths suggested the existence of four populations along the studied area: (1) along Espírito Santo State coast; (2) near the region between Atafona and Macaé coasts; (3) along São Paulo, Paraná and north Santa Catarina States coasts and (4) along Rio Grande do Sul State coast. Probably the differentiation was caused by different environmental conditions observed among regions. Other papers about life history corroborate these results except for the Espírito Santo population about which there is no biological information up to the moment.