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Host location by the Mediterranean fruit fly (Cerutitis cupitatu Wiedemann) was studied from the viewpoint of olfactory and optical properties of certain fruits. Oranges, tangerines, plums, lemons, strawberries and grapes were found to be significantly more attractive for mated females than for mated males of C. cupitutu ( fig. 1). When compared with mated females, virgin females were definitely less attracted to oranges and odourless orange dummies (fi . 2). Dimension has a clearcut influence on the attractiveness of natural fruits for both sexes of the Mejterranean fruit fly. The most attractive rest fruit for female and male C. capitatu (oran e) had the largest size, while the least attractive test fruit (grape) had the smallest size ( fig. 1). The refatively low attractiveness of grapes could be considerably elevated by changin their greenish-yellow colour to purple (table).Since the rate of attraction of female C. cupitutu to oranges, lemons, strawberries and to their corresponding, odourless dummies did not differ significantly ( fig. 2), it was assumed that the fruit volatiles are not essential for the location of those fruits. Moreover, removal of the funiculus and arista from both antennae of female C. cupitutu failed to su press the attraction rate of the former to dummies supplemented by orange volatiles, if comparef with the respective responses of untreated females (fig. 3). The dispensableness of olfactory cues for fruit recognition was corroborated by a similar degree of attractiveness exerted by odourless dummies and dummies impregnated with five gradually increasing dosages (from Electrophysiological recordings from olfactory sensilla on the antenna1 funiculi of female C. cupitutu revealed that the former were particularly responsive to the volatiles of orange, lemon and bergamot fruits applied in dosages increasing from lo-* to lo2 pg (receptor potentials ranging from = 0.8 to = 2.3 mV), while they were less sensitive to the volatiles of apricot (receptor potentials ranging from = 0.5 to = 1.3 mV). The receptor potentials elicited by the above essential oils in olfactory sensilla of the females were = 2.8-3.4 times higher than the receptor potentials induced in corresponding sensilla of the males (fig. 5). It follows that the essential oils of certain host fruits provide olfactory stimuli to female and less so to male C. cupitatu, in spite of the relative insignificance of the former for fruit recognition. Water, which may be emitted as vapour from the leaves and fruits of host trees, was found to induce receptor potentials of similar magnitude (ranging from = 0.3 to = 1.7 mV in response to 0.1-5.0 1. 11 of water) in the olfactory sensilla of both sexes ( fig. 6).to 10' pg) of orange volatiles ( fig. 4).