The North American Amphibian Monitoring Program uses a ranked, categorical calling index to estimate anuran abundance. However, there are few data assessing the assumption that calling index values are accurate indicators of population sizes or to suggest ranges of population sizes associated with specific values of the calling index. This study compared mark-recapture population estimates and call rates to calling index values for Green Frogs (Rana clamitans) in the Upper Peninsula of Michigan. Mean mark-recapture population size estimates and mean calls per minute were greater in ponds with larger calling index values. Similarly, calls per minute increased with increasing population size, although rate of increase declined at high population sizes possibly caused by a higher proportion of noncalling satellite males at high densities. Sex ratios in breeding habitat along the edges of ponds were increasingly male biased as population size increased. These data support the assumption that calling index values are useful indicators of abundance of R. clamitans.