A fungal pathogen that killed adult Diaphorina citri Kuwayama (Asian citrus psyllid) in Florida citrus groves was isolated, characterized molecularly and morphologically and identified as a novel isolate of Isaria fumosorosea (Ifr) (= Paecilomyces fumosoroseus) from the Asian citrus psyllid (Ifr AsCP), but no concentration-mortality or time-response data were obtained. When adult psyllids were sprayed with spores at 28^°C, time response (LT₅₀) values of 111 and 102.5 h at spore concentrations of 1 × 10⁷ and 1 × 10⁸ spores/mL, respectively, were obtained. The LT₉₉, was 167.4 and 174.6 h, respectively, for the 2 spore concentrations. After 192 h, the LC₅₀ value was 6.8 × 10⁵ spores/mL and the LC₉₉ was 2.2 × 10⁸ spores/mL. Ten serial passages Ifr AsCP were carried out on malt extract agar (MEA) and dilute Sabouraud dextrose agar and yeast (SDY) media. The pathogenicity to adult psyllids did not decline, but spore yield declined on the SDY medium and Ifr AsCP consistently produced more spores on SDY than on MEA media. Ifr AsCP was highly pathogenic to the psyllid when healthy adults were exposed to spores from psyllid cadavers stored at -74^°C. Two pilot field trials were conducted in Florida citrus groves to assess methods for confirming infection; Ifr AsCP spores infected immature psyllids when applied at a rate of 1 × 10⁷ spores/mL, but monitoring for infected nymphs required special handling methods.