To study the rate of invasion into brood cells, a group of varroa mites, Varroa jacobsoni Oudemans, was introduced into honeybee colonies, Apis mellifera Linnaeus. For each experiment one or two otherwise comparable colonies was treated, and the effect on the rate of invasion was assessed. The results showed that the rate of invasion increases with the number of available brood cells, and decreases with the size of the honeybee population. This is expected when the rate of invasion is limited because the bees have to carry the mites close to a brood cell for invasion to occur, and only alimited number of honeybees can come close enough to a brood cell simultaneously. In addition, the rate of invasion decreased when young brood was present. This counterintuitive result may arise because the mites prefer young honeybees, young honeybees are likely to stay in the brood nest area, and young honeybees in the brood nest are divided over areas with brood cells that are suitable for invasion and over areas with brood cells that are unsuitable for invasion by mites. Hence, an increase in the amount of young brood, which is not yet suitable for invasion by mites, may decrease the invasion rate. Differences in the period during which brood cells suitable for mite invasion were available to the mites, appeared not to affect the rate of invasion.