Taxic responses may play a role in development of Leishmania in their phlebotomine sand fly vectors. They are possibly responsible for movement of the parasites towards the anterior regions of the gut, from where they would be transmitted to the vertebrate host. A methodology capable to distinguish chemotaxic from osmotaxic responses was described and used to characterise taxic responses in Leishmania promastigotes. These were able to respond to chemotaxic as well as to osmotaxic stimuli. Like bacteria, promastigotes were capable to undergo "adaptation," a phenomenon by which they stop responding to a continuos stimulus. A model capable to explain how a relatively small number of different receptors works to perceive gradients in chemotaxic responses was proposed. According to this model, these receptors possess low specificity and a wide range of affinities varying from high to low. A low specificity makes the same receptor able to bind to a large number of different but structurally related molecules and; a wide range of affinities (considering a population of receptors), implies that the number of receptors "occupied" by attractant molecules along a gradient would go growing step by step.