The present laboratory study focused on studying the biology of a recent invasive insect pest in Egypt, fall armyworm; Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) on leaves of two host plants, maize and castor bean. This pest species has recently invaded Egyptian fields causing unexpected damage to maize crop, so, biology of S. frugiperda was studied under laboratory conditions to estimate its larval instar durations, total larval period, pupal weight, pupal duration, adult emergence%, sex ratio, male and female longevity and fecundity. Obtained results showed that mean incubation period of egg were 2.75 and 3.06 days, while larval durations were 16.26 and 22.03 days and pupal duration lasted 9.93 and 10.40 days, respectively, after rearing on maize and castor bean leaves, respectively. Means of total egg production were 1836.67 and 1562.33 eggs/female by rearing on these two hosts, respectively. Pre-oviposition; oviposition and post-oviposition periods were 3.36, 6.77 and 2.20 days, for females fed during larval stage on maize leaves, opposed to 4.97, 5.80 and 2.50 days when reared on castor bean leaves. Adult longevities were longer by feeding larvae on castor bean leaves than on maize leaves. Also, results indicated that FAW growth indices were higher by feeding on maize leaves than on castor bean leaves. So for rearing S. frugiperda, maize leaves may be used as preferred, but castor bean leaves can be also, successfully used in case of the absence of the former host plant.