Small isolated populations may face an increasing risk of extinction due to the loss of genetic diversity. This increasing risk, though, may be offset by gene flow, provided the population receives an adequate number of migrants per generation. We show that as temporal fluctuation in population size (FPS) increases, so too does the required number of immigrants. This increase in the requisite number of immigrants arises because the ratio of census size to effective population size decreases with increasing FPS. Because all populations fluctuate, our work extends a recent challenge to the widely adopted one migrant per generation rule, which refers to the supposedly requisite number of immigrants. In a sample of 44 animal populations, ˜60% of the populations fluctuated enough to require >10 immigrants per generation to avoid a substantial loss of genetic diversity, and ˜25% fluctuated enough to require >20 immigrants per generation. We thus recommend that estimation of the requisite number of immigrants take into account fluctuation in population size.