Amphibians are greatly affected by diversity loss and recent declines in amphibian populations are well documented. One of the proposed causes of such declines, UV-B radiation, has been reported to induce lethal and sub-lethal effects on different ontogenetic phases of development. Of these phases, the embryo stage is likely the most vulnerable. We used an experimental approach to test for the effects of UV-B radiation on embryo development of the Mexican axolotl, Ambystoma mexicanum. Recently, its natural populations experienced a 60% decline. We conducted experimental trials over the course of a year in which we exposed A. mexicanum eggs to 3 different treatments of varying UV-B ray intensities. Our results indicate that environmental UV-B radiation in Mexico City adversely affects the embryos of the Mexican axolotl at particular times of the year. The embryos exposed to 100% UV-B radiation had strikingly more deformities (mostly blisters and edemas), shorter periods of embryonic development, smaller hatchling size, and experienced increased mortality in comparison with embryos that were partially or totally protected from UV-B rays. The breeding season of A. mexicanum coincides with the time of the year in which the effects of UV-B radiation are most harmful (February through April). The observed negative effects of UV-B rays could act synergistically with chemical pollutants, exotic species, and pathogens to yield the negative population trends currently observed in this endemic Mexican amphibian.