Galimberti,F. 1995. Competizione tra i maschi e selezione sessuale nell' elefante marino del sud (Mirounga leonina) della Penisola di Valdes [Intermale competition and sexual selection in southern elephant seals (Mirounga leonina) of the Valdes Peninsula]. PhD Thesis. Universitˆ degli Studi di Roma "La Sapienza", Roma. The target of my research project was sexual selection by inter-male competition in southern elephant seals (Mirounga leonina) of the Valdes Peninsula (Patagonia, Argentina). I measured various male phenotypic traits, including size, morphology, tenure, behavioural performance. Moreover, I measured the three main components of male individual fitness, i.e., female holding, mating success, and number of females fertilized. I calculated selection pressures using different univariate and multivariate methods, including non-parametric fitness functions, selection differentials and gradients, and path-analysis. At the same time, I studied the structural relationship between male traits, with particular attention to behavioural ones. Structural components of phenotype (age, linear size, weight) were not under any direct selection pressure. On the contrary, they are under indirect pressures due to their effect on behavioural traits, that are the direct target of sexual selection in this population. Secondary sexual traits, including all the components of the proboscis, had small and non significant selection gradients. On the contrary, various measures of behavioural performance in competition, including a cardinal dominance measured at population level, were under strong directional selection. Competition success is determined in part by structural size and in part by basic measures of behavioural activity (interaction rate, aggressiveness), but some of its variance was not explained by the male traits I was able to measure. Male experience may have a significant role in competition among mature males, explaining this residual variance in competition success. The large variation of the local breeding condition (female density, harem size, number of males) seems to have only a slight effect of selection pressures. In the Valdes Peninsula population, deterministic factors seem to effectively determine intra-selection pressures, with the variation in male phenotype explaining the majority of the variation in competition and breeding success.