Makisterone A is the predominant ecdysone in the 96 +/- 4-hour-old embryo of the large milkweed bug and it is the first molting hormone with a C-24 alkyl substituent of the side chain to be isolated and identified from an insect. In addition, unknown compounds that may represent other C28 ecdysones were detected in very low concentrations. The milkweed bug could well possess a biosynthetic-metabolic pathway for C28 molting hormones instead of or in addition to known pathways for the C27 ecdysones.