The goal of this research was to describe developmental rates, reproductive rates, and infestation patterns of Aphis craccivora Koch on alfalfa (Medicago sativa L.). All studies were conducted on the susceptible cultivar OK08 using aphids reared from collections made in Oklahoma. To determine thermal requirements for growth of A. craccivora, development from birth to adult was recorded at 7.2, 12.8, 18.3, 23.9, and 29.4°C. The same constant temperature treatments (except for 7.2°C being raised to 8.3°C) were used to assess the influence of temperature on reproductive rates. Within-plant distribution patterns were determined by infesting three stems on each of 24 plants and recording numbers of A. craccivora on leaf blades, petioles, and internodal stems sections at 2-d intervals through 10 d after infestation. Aphid counts were analyzed to determine significant differences among node parts (leaf blades, petioles, and stem sections). The developmental threshold temperature for A. craccivora was calculated to be 7.1°C, and the thermal constant for development from the first instar to reproducing adult was 100 DD (°C). The optimal temperature range for reproduction on alfalfa was 18–24°C, with a mean of 82 nymphs produced per female. From the initial infestation of three apterae per stem, numbers increased to a mean of 510 per stem after 10 d. Plant profiles showed that the greatest numbers of aphids were located in middle and lower portions of the plant canopy. On all sampling dates, the proportion of aphids on internodal stem sections was significantly greater (P < 0.05) than on petioles and leaf blades.