Mate choice is one of the strongest factors driving evolution, yet little is known about the costs females incur by selecting among potential mates. The energetic cost of male assessment is typically assumed to be low, particularly in lekking species where male territories are often tightly clustered; however, these costs have not been directly quantified in any species. I examined the costs of female choice in lekking Galápagos marine iguanas ( Amblyrhynchus cristatus) using a combination of heart-rate bio-logging and continuous behavioral observation. The results indicate that females expend substantial energy visiting and assessing potential mates. Females prefer territorial males that sustain the highest rates of head-bobbing display; however, assessing these males appears to be especially costly. Females that spent more time on the territories of these high-quality, high-activity males expended more energy on mate choice, lost more mass, and showed a trend towards producing smaller follicles. Choosy females also face a reduced probability of survival if El Niño conditions occur in the year following breeding. Investment in mate choice varies both between years (as a function of resource availability) and among females employing differing choice tactics. Finally, I discuss several unusual adaptations of marine iguanas (including biennial and condition-dependent follicular atresia and heterospecific eavesdropping) that likely provide substantial energetic benefits to these highly energetically constrained reptiles. The finding that female choice can carry significant costs suggests that the genetic benefits that lek-mating females gain through mating with a preferred male may be higher than previously predicted.