The behaviour of honeybees foraging in an artificial flower patch was studied under a variety of experimental conditions. Honeybees associated food with a flower colour and remained constant to that colour. Constancy was on an individual basis, with some bees constant to yellow and some to blue flowers. Individual bees were approximately 97'5% constant to a colour, and the occasional visits to a flower of another colour did not change their constancy to the original colour. The presence or absence of odour stimuli did not change the bees' foraging behaviour, nor did a change in the volume of nectar-per-f1ower obtained. The foraging behaviour described is potentially a strong isolating mechanism for plants. Since changes in flower colour or morphology can be accomplished by single gene mutations, the results present conditions in which plant speciation may occur much more rapidly than current dogma predicts. Also, sympatric speciation in plants would not be as unlikely as previous studies suggest. Computer simulations showed that the gene leading to ethological isolation and a gene under differential selection in 2 sympatric microhabitats need not be linked in order for divergence in allele frequencies to occur.