Susceptibility of Marsh grapefruit to laboratory infestation by Anastrepha suspensa was determined each month between October 1982 and May 1983. Criteria for susceptibility included comparisons of oviposition rates and fly developmental success. Five times as many flies were produced per fruit in May as in October. Fruit exposed in May received ca. 2.5X as many eggs as in October, and egg-adult success was ca. 2X greater in fruit exposed in May (8.0% vs. 3.9% in October). Selected chemical parameters (peel oil linalool concentration and peel sugar concentration) monitored throughout the season did not correlate with the observed increase in susceptibility of senescent fruit.