Ant—aphid interactions are well-studied mutualisms, but surprisingly, the intraspecific variation in the degree of ant attendance of aphids has been less appreciated than interspecific variation. Honeydew composition is a key factor in determining aphids' traits in relation to the intensity of ant attendance, and composition may be altered by endosymbiotic bacteria. Here, we evaluate relationships among the intensity of ant attendance, honeydew composition, and aphids' endosymbionts. We found differences in the intensity of ant attendance and endosymbiont composition between clones of cowpea aphids (Aphis craccivora Koch). To investigate whether aphid genotype, endosymbionts, or both, influence honeydew composition, and the relative influence of these factors on the intensity of ant attendance, a secondary symbiont of the aphid, Arsenophonus, was removed from the clone exhibiting a lower degree of ant attendance. Then, the sugar and amino acid composition of honeydew were compared between different aphid genotypes and between same genotype with and without Arsenophonus. The experiments demonstrated that there was a difference in the sugar composition of honeydew between clones, but that Arsenophonus did not influence this difference. Furthermore, the intensity of ant attendance of the clone did not change after removing Arsenophonus. These results suggest that variation in the degree of ant attendance of this aphid is caused by aphid genotype, not by endosymbionts.