Oganophosphorus (OP) and carbamate (CB) insecticidesare widely used and havea variety of lethal and sublethal effects on nontarget wildlife, primarily through cholinesterase (ChE) inhibition. To assess possible exposure to anti-ChE compounds in wading birds, we monitored breeding colonies in northeast U.S. estuaries (Boston Harbor, MA; New York Harbor, NY; Nantucket Sound, MA; Delaware Bay, DE; and Rehoboth Bay, DE) from 1991 to 1996. We documented serum ChE activities in black-crowned night-heron (Nycticorax nyticorax), cattle egret (Bubulcus ibis), snowy egret (Egretta thula), little blue heron (E. caerulea), and glossy ibis (Plegadis falcinellus), and we investigated factors known to affect ChE, including age, nutritional and immune status, location of the colony (estuary), and exposure to ChE-inhibiting compounds. Exposure to anti-ChE compounds in all species was supported by at least one of the following: positive pyridine-2-aldoxime methochloride (2-PAM; OP) and/or spontaneous (CB) ChE reactivations, negative or nonsignificant age correlations in most species, or regional land-use patterns. We observed negative or nonsignificant relationships between ChE and age in most species. Only glossy ibis showed the age-related patterns of ChE activity observed in other altricial species. Of the remaining potential explanatory factors, location (estuary) but not nutritional or immune status was the only one having a significant relationship with ChE activity (p < 0.024). Significant differences among colonies were consistent with surrounding land uses, specifically active agriculture. We conclude that extensive monitoring of serum ChE in wildlife can identify locations of exposure and provide reference data for wildlife pesticide-risk assessment.