A study was made of the metabolism of β-sitosterol by nymphal German cockroaches (Blattella germanica (L.)) held for 42 days on a synthetic diet containing 0.2% of the H3-labeled sterol. Analysis of the total H3-compounds from the cockroaches indicated about 93% was present as free sterols and only 6% as esters. When the ▵5 fraction, which accounted for about 97% of the total H3-sterols in the cockroaches, was analyzed by gas chromatography and/or reverse isotope dilution, about 80% was found to be cholesterol and only about 7% behaved like unchanged β-sitosterol. These data indicate that the German cockroach has available the biochemical mechanism for the removal of the 24-ethyl group from the β-sitosterol side chain.