We offer two hypotheses incorporating habitat patchiness to account for microtine cycles. Both emphasize the role of immigrants in generating microtine cycles. The first, the predation hypothesis, assumes that voles and grass are engaged in predatorvictim oscillations and that the voles are equally proficient predators in habitats of differing productivity. It is shown how this may lead to cyclic populations in richer habitats. The second hypothesis, the phenological hypothesis, assumes that voles track their resources by responding to stimulants and inhibitors in the plants which signal periods of plant growth. Because of patchiness, the optimally sensitive vole in one area is different from one in another. Immigration prevents precise local adaptation, and voles sometimes breed at inappropriate times. This overloads food quality causing its decline, followed by vole decline and selection against sensitivity to the plant stimulants. Cycles result. Comparison of the hypotheses with previous results suggests the phenological hypothesis is more probable.