The author observed mature Rubus guyanensis Focke fruits along with immature ones. Observable morphological differences between two morphs, namely the “Oblong-acuminate” and “Cordate-lanceolate” fruits, were not found. The fruit of Rubus guyanensis “Cordate-lanceolate” morph is tiny in contrast to those of other Rubus species. The fruit of R. guyanensis is black when physiologically mature. The author recorded the weight, width, and length of 70 “Oblong-acuminate” morph Rubus guyanensis immature reddish fruits using digital calipers to the nearest 0.1 mm. The author also recorded the weight, width, and length of 75 “Cordate-lanceolate” morph Rubus guyanensis mature fruits. The mean length, width, and weight of the immature “Oblong-acuminate” morph were 7.36 mm, 6.18 mm, and 0.19 g, respectively. The mean length, width, and weight of the mature “Cordate-lanceolate” morph were 10.84 mm, 8.79 mm, and 0.51 g, respectively. The author analyzed the fruit length-weight and width-weight relationships in both morphs. Mean pyrene length was 2.71 mm in the “Cordate-lanceolate” morph and 2.22 mm in the “Oblong-acuminate” morph. The pyrene bears a testa that is rugged and has an ornamentation that has high ridges and, in some areas, lacunas or lower areas. The pyrene border has a flat disk-like structure that surrounds about half of the pyrene.