In this thesis I address several fundamental questions in sexual selection and mate choice theory, using the sandfly Lutzomyia longipalpis. I show that L. iongipalpis satisfies the criteria for a lek-breeding species. Males aggregated at sites near hosts which females visited to mate and feed, but a male's access to a resource did not appear to detennine his mating success. Females were free to reject unwanted males and male mating success was typically skewed. Field data suggest that the mating system may have evolved because of female preferences for larger leks, coupled with a hotspot-type mechanism: male distribution was correlated with resources availability, while females distribution was correlated with lek size. In the laboratory, females preferentially mated with middle-aged males. Within this age class, mating success was correlated with increased amounts of pheromone and increased investment in wing-fluttering. Across age classes, females appeared to gain a direct fitness payoff from their choice of mate through increased probability of fertilisation, but the presence of other mechanisms was not investigated. By testing simultaneously the predictions of current models using similar age males, I was able to assess their relative importance for the maintenance of female choice. I found weak evidence to support direct benefits: females that chose to mate with successful males survived longer post-oviposition than females with less successful mates. This did not translate into increased total longevity or increased fecundity. I found no evidence in favour of good-genes models: offspring of preferred males did not survive longer than offspring of less preferred males, nor were their daughters more fecund. Male attractiveness was, however, heritable: sons sired by preferred males achieved higher mating success than sons of less preferred fathers. These results suggest that a Fisherian mechanism is in part responsible for the maintenance of female mating preferences in L. iongipaipis.