Coffea arabica L. faces serious disease susceptibility problems, favored by the low genetic variability of its commercial cultivars; therefore, it is important to study different sources of variation that are useful in genetic improvement. The objective of this study was to irradiate C. arabica-1 seeds of the Geisha, Oro Azteca and Marseillaise varieties with gamma rays to determine the median lethal dose (LD50) and evaluate their physiological response in terms of germination, survival, plant height (PH), stem diameter (SD), height to first leaf pair (HFLP) and leaf area (LA). The Transelektro LGI-01 irradiator was used, with a dose rate of 752.76 Gy·h-1. The irradiation doses evaluated were 0, 100, 200, 300, 400 and 500 Gy. The experiment was established under a completely randomized factorial design with two factors (variety, with three levels, and irradiation dose, with six levels) and three replicates (225 seeds per replicate). The germination record was made 20 days after sowing. The remaining variables were evaluated 120 days after sowing. The results showed that germination, survival, PH, HFLP, SD and LA were significantly negatively affected by gamma irradiation from the 200 Gy dose in all three varieties. The LD50 was 70 Gy for the Geisha variety, 85 Gy for Marsellesa and 90 Gy for Oro Azteca. Doses lower than 100 Gy can be used in breeding programs for C. arabica-1.