Biology and seasonal dynamics of Snellenius maculipennis (Szepligeti), a solitary larval endoparasitoid of castor semilooper, Achaea janata (Linnaeus) were studied in the laboratory and field. The egg + larval, pupal and adult periods lasted 6.25 [plus or minus] 0.96, 5.08 [plus or minus] 0.90 and 3.5 [plus or minus] 1.02 days, respectively. Under laboratory conditions the parasitoid completed its life cycle in 14.83 [plus or minus] 2.88 days. Host larvae showed reduced feeding in the first 5 - 6 days after parasitisation till the exit of the grub from the host body for pupation. After parasitoid exited, the host larvae survived up to 4 - 6 days but exhibited complete feeding cessation leading to drastic reduction in growth. The parasitoid was found active from August to October and the peak parasitism across three years ranged from 68 to 96 per cent. Three years pooled data on larval counts and number of parasitized larvae per plant recorded at weekly interval revealed a linear relationship between them. Two species of hyperparasitoids, viz. Mesochorus pilicornis (Cameron) (Hymenoptera: Ichneumonidae) and Brachymeria secundaria (Ruschka) (Hymenoptera: Chalcididae) were recorded on S. maculipennis for the first time. The extent of hyperparasitism was between 26 and 38 per cent during August to October. The nature of exit hole on the cocoons of S. maculipennis was described to record the level of hyperparasitism under field conditions. Compatibility study with A. janata granulosis virus revealed that 69 per cent of parasitized larvae collected from virus treated plots showed presence of viral infection. The potential of S. maculipennis in the natural biological control of A. janata and the need for its conservation have been discussed. Andhra-Pradesh, Hyderabad-India none