It has been observed that biological systems have different sensitivities to various radiation qualities. Moreover, the relative response depends on the dose level of the applied radiation. In this context the radiosensitivity of Med-fly, Ceratitis capitata Wied., cells, to gamma-rays and neutrons, has been tested by observing cell population growth and mortality. Fast neutron doses ranged between 1.5 and 25 rad, a region of particular interest in radiation protection studies; the 60Co gamma-ray dose values were between 300 and 5 000 rad. The experimental results show a much higher radiosensitivity of these cells to fast neutrons than to gamma-rays, and indicate a dependence of the RBE values on the neutron-dose levels. Furthermore, there was a certain dependence on radiation quality for the morphological damage. Dose-effect relationships are analyzed for both neutrons and gamma-rays and the results are discussed.