The reproductive biology and size structure was determined for Lethrinops gossei, one of the most abundant fish species contributing 26% of the catch in the demersal trawl fishery of the Southeast Arm of Lake Malaŵi. Breeding seasonality was analyzed on the basis of monthly variation of gonado-somatic index and gonadal appearance, while sexual maturity was determined by fitting the logistic model to the proportions of sexually mature individuals. Length frequency data were used to characterize size structure of the population and to derive a selection ogive for the species based on the currently used cod-end mesh size of 38 mm in the fishery. Results suggest that even though mature individuals were encountered all year round, the main breeding events for L. gossei occur in March and September. It was also determined that the size at which 50% of female L. gossei reached maturity (L50%) was 137 mm total length, while 100% sexual maturity was reached at 170 mm total length. The size at first maturity was 122 mm total length, while the steepness parameter of the maturity ogive, (δ) was 7 mm. Distribution of sizes in the sample was roughly symmetric about a mean of 145 mm total length, while the size at 50% selection was 120 mm total length. These results are discussed in the context of their implications on the management of the demersal trawl fishery in the Southeast Arm currently dominated by L. gossei, given the misalignment of the closed season currently in October to December and the inadequacy of the cod-end mesh size of 38 mm. The main conclusion of this study was that the current cod-end mesh size in the fishery is too small and should be adjusted to prevent growth overfishing. A proposal is made for a closed season for the trawl fishery from March to April.