• Photopic and scotopic increment thresholds were measured at selected visual field positions for 39 patients suspected of having glaucoma, 39 patients with diagnosed glaucoma, and 31 agematched normal control subjects. Stimuli were presented at equal-eccentricity pairs of positions just above and below the nasal horizontal meridian. Both photopic and scotopic thresholds were elevated significantly for both the suspect and glaucoma groups. The average photopic and scotopic threshold elevations were the same for the suspect group, but scotopic threshold elevations were substantially greater than photopic threshold elevations for the glaucoma group. In a more detailed analysis of the glaucoma group data, local and diffuse components of the photopic and scotopic threshold elevations were operationally defined. The analysis revealed that localized photopic and scotopic scotomas were of equal depth, but diffuse scotopic defects exceeded diffuse photopic defects by a factor of 2:1 log units. These findings suggest that not all ganglion cell types are equally susceptible to glaucomatous damage and also point to scotopic perimetry as a potentially sensitive test for early glaucoma detection.