Feeding behavior of Melanaphis sacchari Zehntner (Hemiptera: Aphididae) was studied on sugarcane, Saccharum spp. (Poaceae), cultivars HoCP 91-555 (resistant), LCP 85-384 (moderately resistant), and L 97-128 (susceptible) using the electrical penetration graph (EPG) technique. Constitutive concentrations of total phenolics and available carbohydrates, water potential at the whole-leaf tissue level, and free amino acids (FAAs) in phloem sap extracts, and in honeydew produced by aphids fed on L 97-128 and HoCP 91-555 were determined. Cultivar did not influence time for M. sacchari to access phloem sieve elements. Total time in sieve elements was ca. two-fold greater on L 97-128 than on HoCP 91-555, whereas it did not differ from LCP 85-384 in either cultivar. The mean duration of individual events associated with phloem sap ingestion was ca. 50% shorter on both HoCP 91-555 and LCP 85-384 than on L 97-128. Although cultivar effects were not detected for levels of total phenolics, available carbohydrates, and water potential, two free essential amino acids, histidine and arginine, were absent from phloem sap in HoCP 91-555. Two free essential amino acids, leucine and isoleucine, and two free non-essential amino acids, tyrosine and proline, were absent from honeydew of aphids fed on HoCP 91-555. These results suggest that despite apparent biosynthesis of some FAAs, the absence of important FAAs in the phloem sap of HoCP 91-555 and the inability of M. sacchari and its endosymbionts (e.g., Buchnera) to derive specific free essential and non-essential amino acids from other ingested molecules, possibly along with other unidentified factors, underlie the pest's decreased phloem sap ingestion and consequently reduced growth potential on HoCP 91-555.