Two new agelastatins, C (3) and D (4), along with the known compound (-)-agelastatin A ii), were isolated from the Indian Ocean sponge Cymbastela sp. The structures were determined by analysis of spectroscopic data and chemical correlation of 4 with a common derivative 5, also prepared from 1. Agelastatin A (1) exhibited potent activity against brine shrimp (LC50 1.7 ppm) in addition to insecticidal activity against larvae of beet army worm, Spodoptera exigua, and corn rootworm, Diabrotica undecimpunctata.