A patch assessment model is used to describe the behaviour of bumblebees foraging at inflorescences with flowers arranged in vertical rows. The bees start at the bottom and tend to move upwards from flower to flower. This results in a positive correlation between nectar volumes of flowers belonging to the same row. Therefore, a bee should continue upwards if it finds a rich flower, but change to another row if it finds a poor. When a poor flower has been found in all rows, the bee leaves the inflorescence. The bees distinguish between rich and poor flowers by means of a threshold volume, which increases with the mean nectar volume in the area. Laboratory experiments confirmed the hypothesis, as did field observations, except on one point which is discussed. The nectar distribution at inflorescences was such that a bee following the movement rules obtained a higher gain than a bee searching randomly. A model is presented which explains how the optimal threshold can be found, and it is discussed if the observed threshold function is optimal. The models are used to make a number of predictions concerning the foraging behaviour of bumblebees, especially effects of arrival point, mean nectar volume, inflorescence size and foraging costs.