The chemistry of fifty lakes in the Sudbury, Ontario region was studied by float equipped aeroplane in August, 1973. Conductivity, alkalinity, chlorophyll a, NO3, PO4, SiO2, SO4, F, Cl, Br, Ca, Mg, Na, K were measured as well as “labile” metals. Conductivity ranged from 28 to 264 umho cm−1 with a distinct bimodal distribution related to the presence or absence of calcareous material. Twenty-nine percent of the study lakes showed pH’s <5.5, a value below which reproductive success of fish is questionable. A calcite saturation index (CSI) was defined to predict lakes with low pH, lakes susceptible to pH depression, and lakes with sufficient buffering to provide protection against pH depression from atmospheric fallout. Twelve (24%) of the study lakes have CSI’s that make them critically acid and an additional fifteen lakes (30%) are susceptible to pH depression with continued atmospheric loading. The program utilizing a field laboratory and a float plane was low cost and permitted maximum data collection in the time available. Further, the rapid return of data in the field allowed ongoing verification and modification of the program.