The entomopathogenic fungus Isaria fumosorosea Wize (PFR97^® strain, Certis USA, Columbia, MD) was tested as a mortality agent of the cycad aulacaspis scale, Aulacaspis yasumatsui Takagi, at 2 temperatures in the laboratory. First instars of A. yasumatsui were treated with 1 of 4 concentrations of I. fumosorosea (5.4 × 10⁷, 9.9 × 10⁶, 6.4 × 10⁵, or 1.8 × 10⁵ blastospores/ml of water) or a water only control. Following treatment, insects were held at either 20 or 30°C. The highest concentration treatment resulted in the highest mean infection rate at 8 d postapplication (73 ± 4.2% at 30°C; 84 ± 3.7% at 20°C). However, there was no interaction between blastospore concentration and temperature for infection rate. The lowest mortality rate was obtained with the highest concentration of blastospores under 20°C (13 ± 3%). The LC₅₀ at 20° and 30°C were 6.1 × 10⁶ and 5.3 × 10⁶ blastospores/ml, respectively. The LT₅₀ was lower at 30°C than at 20°C for the 3 highest concentrations. The radial growth of the fungus on potato dextrose agar 20 d after inoculation was 37% greater at 30°C than its radial growth at 20°C. These results indicate that I. fumosorosea may be a new biological control weapon for suppressing infestations of cycads by A. yasumatsui. This is the first report of I. fumosorosea being evaluated to infect an armored scale insect.