Postharvest control of green mold produced by Penicillium digitatum in citrus packing houses in Peru is mainly based on the use of imazalil fungicide (IMZ). This has increased populations of resistant biotypes of the pathogen. The objective of this work was to evaluate the resistance of 15 isolates of P. digitatum to IMZ following three steps. Isolates were collected from ‘W. Murcott’ and ‘Nova’ mandarin fruits with the presence of the disease eliminated during packing activities. First step consisted to evaluate 15 isolates for their resistance level, at 1, 2 and 5ppm of imazalil (PDA + IMZ). Fourteen survived to 5ppm and were considered highly resistant. Second step consisted to compare the infection speed on ‘W. Murcott’ fruits. Fruits were previously clean by immersion for 60 s in a solution with 1% of sodium hypochlorite before being infested with the 14 imazalil-resistant P. digitatum isolates. Severity was graded after 15 days at 20±2°C, measuring the ratio of the sporulation and rotting. Three isolates showed the highest percentages of severity and followed the third step that consisted in evaluate conidia germination. Conidial suspension of the three isolates was carried out until 200 conidia were observed per visual field and then they were taken to slides with 1 drop of PDA which were kept at 25°C. Only one isolate was significantly higher than the other two isolates whose percentage of germination was 97.5% after 22 h of assembly. Once the most resistant isolation to imazalil has been identified, control trials using alternative methods that prevent appearance of resistant green mold can be undertaken.