Detailed field studies and petrographic analyses of Chaetetes-bearing sections reveal Chaetetes in the presence of Profusulinella in the La Tuna and Berino Formations (Morrowan-Atokan) of the Magdalena Group in the Northern Franklin and Hueco Mountains, Texas and New Mexico. Chaetetes and non-Chaetetes lithotopes are characterized by a diverse and abundant assemblage of benthonic organisms typical of a clear, well-illuminated shallow-water carbonate platform of the late Paleozoic. Two chaetetid growth forms with apparent écologie significance were observed: shingle-form Chaetetes and club-form Chaetetes. Shingle-form Chaetetes are characterized by numerous increases or decreases in width of the upward-growing colony, and commonly form anastomosing complexes of colonies. Club-form Chaetetes evidence subtle changes in width, emphasizing growth along the vertical axis; clustering of solitary club forms is common. Both growth forms are excluded from high-energy shoaling environments. Shingle-form Chaetetes are restricted to the base of lithotopes characterized by prolific growths of phylloid algae. Cuneiphycus, and tubular organisms. Competition with these organisms for substrate space apparently limits Chaetetes distribution on the platform. Common, erratic growth discontinuities in shingle forms and associated burrow-churned wackestones and packstones imply frequent fluctuations in the physical and biological environment. Stagnant, poorly oxygenated conditions on the sea floor limit solitary clubform Chaetetes to the base of a laminated bioclastic mudstone lithotope. Relatively stable conditions in quieter, deeper water are suggested by the associated discretely burrowed, well-laminated mudstones and wackestones.