Subpopulations of primary sensory neurons in mammalian dorsal root ganglion (DRG) exhibit carbonic anhydrase (CA) activity. To identify these subpopulations in DRG cells of mouse and chicken, the reliability of the cytochemical localization of the enzyme requires that several conditions be fulfilled: Preservation of the enzyme activity in glutaraldehyde-containing fixative; accessibility of the cytoenzymatic reaction throughout 20-micron thick Vibratome sections; retention of the reaction product in situ during OsO4 post-fixation; specificity of the cytoenzymatic reaction for CA activity as corroborated by the immunocytochemical detection with antibodies anti-CA II in mouse DRG; strict correlation between the CA activity and the cytological characteristics in a given subclass of neurons. On the basis of these criteria, it is concluded that the CA activity may be used as a cell marker to identify cytologically defined neuronal subpopulations and their axons in mouse DRG. In chicken DRG, CA activity is not consistently expressed in a given subclass of ganglion cells and their axons. Hence, it is assumed that the expression of CA activity by DRG cells in chicken is modulated by functional or environmental conditions.