Oxygen isotopes can record water depth changes because of the relationship between depth and temperature, and ice volume changes by its relation to the isotopic composition of sea water. Because of this, oxygen isotopes provide an excellent means of studying the conditions responsible for the formation of cyclical sedimentary deposits in the late Paleozoic. We have determined the isotope stratigraphy of two late Pennsylvanian units in north Texas, the Necessity Shale and the Colony Creek Shale, after having carried out detailed stdies of the depositional environments andfossil biotas in these units. Isotope data were obtained from fibrous calcite in the shells of the brachiopod Crurithyris planoconvexa, which is present throughout both shales. Preservation of original isotopic composition in fibrous calcite is indicated by preservation of original shell microstructure, absence of cathodoluminescence, low iron and manganese concentrations, and high sulfur concentration within shells. These characters were determined by plain light and cathodoluminescence microscoPy, microprobe, and SEM-EDS. In contrast, prismatic calcite from the outer shell layer and some outer margins of the fibrous shell layer are diagenetically altered. This alteration is recognized by visible cathodoluminescence, high iron and manganese concentrations, and low sulfur concentration. The two cycles studed can be divided into as many as four depth zones based on sedimentology, fossil biota, and position within transgressive and regressive sequence. From shallowest to deepest, they are the myalinid zone, fusulinid zone, ammonoid zone, and gondolellid zone. The deepest faunal zone, defined by the occurrence of the conodont Gondolelia, correlates with the maximum 8180 in the fibrous calcite of C. planoconvexa. This indicates that the 8180 of the brachiopods is recording a decrease in emperature associated with deepening of water, and that temperature effects are the major component of the oxygen isotopic signal present in the cycles. Using modern tropical ocean temperature profiles as an analogue for interpretation of the 180 stratigraphy, the depth change estimated for deposition of the Necessity cycle in the Brownwood area of Texas is at least 70 m. INTRODUCTION