The effects of carbon dioxide concentration on the molting activity of Amblyomma americana (L.), and Amblyomma maculatum Koch were investigated. In both species, replete nymphs held in humidity chambers at a high carbon dioxide concentration (ca. 900 pp,) had a 2-4 day delay in peak ecdysis when compared to nymphs held at a low carbon dioxide concentration (ca. 400 ppm). In contrast, replete larvae showed no apparent delay in peak ecydsis [sic.] as a result of increased carbon dioxide concentration. Percent total eclosion was not affected by carbon dioxide concentration in either nymphs or larvae.