A review of the literature dealing with irritability and other behaviour phenomena shown by anopheline mosquitos in relation to house treatment with residual insecticides has indicated the need for a re-appraisal of terms and criteria. A general term, "behaviouristic avoidance", has been proposed to deal with all those aspects of behaviour in which mosquitos, irritated by sub-lethal contact with insecticide-treated surfaces, can escape apparently unharmed from treated houses.A critical examination of relevant figures has as yet failed to reveal any convincing proof as to the existence of behaviouristic resistance-that is, cases in which the phenomena of irritability and avoidance are reported to have appeared or to have been intensified only as a result of continued insecticide pressure.Various criteria are discussed in relation to establishing the existence of "behaviouristic avoidance" in a species, and in deciding whether this irritability is natural (i.e., "protective avoidance") or whether it could be attributed to an intensification of irritability under continued insecticide pressure (i.e., "behaviouristic resistance").