U.S. credit unions serve 93 million members, hold 10 percent of U.S. savings deposits, and make 13.2 percent of all non-revolving consumer loans. Since 1985, the share of U.S. depository institution assets held by credit unions has nearly doubled, and the average (inflation-adjusted) size of credit unions has increased over 600 percent. We use a non-parametric local-linear estimator to estimate a cost relationship for credit unions and derive estimates of ray-scale and expansion-path scale economies. We employ a dimension-reduction technique to reduce estimation error, and bootstrap methods for inference. We find substantial evidence of increasing returns to scale across the range of sizes observed among credit unions, suggesting that further industry consolidation and growth in the average size of credit unions are likely.