The littoral macrozoobenthos (MZB) of two low acid neutralizing capacity (ANC = <50 .0 Β΅eq 1 -1 ) northeastern Pennsylvania lakes (treatment and reference) were sampled each ice free season, 1984 through 1988, to evaluate response to limestone addition . Following addition of 100 tons agricultural limestone (CaCO3) to the treatment lake in February 1985, mean annual ANC rose from 20 Β΅eq 1 -1 to 668 Β΅eq 1 -1 then decreased . In October 1987, ANC in the treatment lake was decreasing, so 15 additional tons of limestone were added . Pre-limestoning there were few significant (p<0 .05) differences between MZB assemblages in the two lakes, but 4 years post treatment the following differences (p<0.05) were detected in the treatment lake : more Chironomidae, Ephemeroptera, Odonata, Hyalella azteca, total MZB numbers and higher Mollusca wet weight than in the reference lake . From 1984 to 1988 in the treatment lake the annual means of the following community characteristics increased (p<0 .05) : totals of Chironomidae, Ephemeroptera (including Caenis spp .), Odonata, Trichoptera, Sphaeriidae, collectors and MZB numbers ; Chironomidae and Ephemeroptera taxa richness and total taxa richness ; total and Mollusca wet weight . However, the treatment lake ecosystem is neither self-sustaining nor stable because limestone must be added every 2 to 3 years to maintain ANC above 50 Β΅eq 1 -1 In the reference lake annual means of ANC, specific conductance, calcium and magnesium decreased (p<0 .05) while Secchi depth increased (p<0 .05), suggesting continuing acidification.