The efficacy of a new semi-synthetic avermectin derivative Methylamine avermectin (Radical 0.5% EC) was evaluated on the larvae of the Egyptian cotton leafworm, Spodoptera littoralis (Boisd): (Lepidoptera: Noctuidae). The 2nd instar larvae was more sensitive than 4th instar larvae, the determined LC50 for 2 nd instar larvae was 0.021 ppm. From the obtained results it was clear that the larval duration, pupal duration, pupal weight, pupation percentage, adult emergence and the number of laid eggs of the 2nd instar larvae treated with the LC50 of methylamine avermectin were decreased significantly compared to their control. While, male and female longevity insignificantly affected compared to control. On the other hand, male albino rats were administered oral doses equivalent to 1/10 or 1/40 of the LD50 value (45mg/kg body weight) for 21 days (day after day), since, methylamine avermectin affect on the biochemical parameters of liver function (i.e. aspartate aminotransferase activity, alanine amino transferase activity, albumin, cholesterol and triglycerides levels), kidney function (urea and creatinine concentration), testes function (testosterone).