Protein-rich diets are known to promote ovarian and egg development in workers of the honeybee, Apis mellifera, even in the presence of a queen. Since the main source of protein for honeybees is pollen, its quality and digestibility might be important dietary factors determining reproductive capacity. We have compared the effect of two types of pollen-sunflower, Helianthus annuus, and aloe, Aloe greatheadii var davyana-on ovarian development in A. mellifera scutellata workers. Under queenright conditions in the field, worker bees exhibited greater ovarian development when feeding on aloe pollen than on sunflower pollen. In their midgut, we observed higher extraction efficiency for aloe (80%) than for sunflower (69%) pollen. This may be attributed to the morphology and size of the two kinds of pollen grains and explains, together with the high protein content of aloe pollen (32% dry mass in bee-collected pollen) compared to sunflower pollen (15%), why aloe pollen promoted higher ovarian development. However, in the laboratory workers sustained on aloe pollen had significantly less-developed ovaries and higher mortality than those fed sunflower pollen. These detrimental effects may be due to an unbalanced protein:carbohydrate ratio. We discuss the effects of unbalanced diets on the physiology and ecology of honeybee reproduction.