The diet of the Guianan Cock-of-the-Rock (Rupicola rupicola) was studied in a pristine lowland rainforest, near an inselberg in French Guiana. Droppings were collected under a nest and observations were conducted on a radio-tagged female. Nestlings were mostly fed fruit, but not exclusively so, as insects, small snakes, and lizards were also delivered to the chicks. The characteristics of the fruits consumed, and of the fruiting plants, were given special attention. The fruit diet is highly diversified : 65 species belonging to 31 families were recorded as ingested in a little more than one month. Remains of fifty-two species belonging to 26 families were identified under the nest itself. A quantification by feeding units (representing what the bird actually takes on the plant) shows that three quarters of the diet were contributed by only a quarter of the fruit species. Two thirds of the feeding units were contributed by only four plant families. Half the number of fruit species and two thirds of the feeding units were drupes, generally 10 to 20 mm in breadth, with seeds 5-1 5 mm wide. 83 % of the fruits were uniformly coloured. Taken together, black and red fruits were three times more abundant than all the others. The pulp of more than two thirds of the fruits amounted to at least three fifths of the fruit itself. A large proportion of the fruits consumed were those of canopy trees and lianas. More than three quarters of the fruits were from plants with large simultaneous crop production. These fruits were plucked out on the wing by the adult female which patroled a home range of 9.7 ha when feeding her brood, going from one fruiting tree to another. These results are compared with the scanty littérature data on the diet of the two Rupicola species. Cocks-of-the-rocks cannot be considered as specialized frugivores, sensu Snow (1981). The assumed coevolution between cocks-of-the-rocks and plants is briefly discussed.