The fate of drug resistant mutations depends on factors, which we may be able to control, such as the rate and pattern of drug use. However, it also depends on factors over which we have no control, the most important of which is the biological cost that resistance imposes on the fitness of parasites. Drug resistance mutations are known to disrupt the parasite’s metabolism, generating fitness costs. In drug-treated hosts, these costs are largely compensated by the benefits conferred by the resistance. In untreated hosts, however, the magnitude of these costs will determine whether these mutations will persist and spread in the population. Current views about the costs of drug resistance in malaria parasites are almost entirely based on data regarding parasitic infections in the vertebrate host. The costs of resistance in the mosquito vector have either been ignored entirely or been given only cursory attention. I will present data and ideas arising from a new project that aims to investigate the costs of drug resistance over the whole life cycle of Plasmodium.