The larvae of the cossid moth Semitocossus johannes (Staudinger) were found to utilize the native woody tree-like milkweed Calotropis procera (Aiton) W.T., Aiton. at Al Muzahimiyah, Al Kharrarah Park, Kingdom of Saudi Arabia. The life cycle of this moth is apparently bivoltine with overlapping emergence. Adults were captured from early April to late June as mean air temperature reached 27–35°C. Another adult activity peak occurred in early October, when air temperatures again ranged 24–34°C. Adult females laid eggs singly or in small clusters on the inner base of woody branches. Eggs hatched on the average of 58 (45–70) days under laboratory conditions. Measurements of the head capsule width and body length were used to recognize size classes I–VI; size classes III–V were present throughout the year. Most larvae tunneled larger branches at the lower portion of the main trunk of the plant. Pupation occurred within the galleries of mature larvae near a prepared exit hole. Before adult emergence, the active pupae moved partially through the hole. Larvae of S. johannes can cause severe damage and death to infested C. procera plants. Numerous larvae (47–192) could be collected from a single infested plant.