SUMMARYMost Dutch lakes are small and shallow, resulting from peat dredging since the late 18th century. However, deep lakes have appeared recently owing to sand digging. Limnological features of one such lake, Wijde Biik (N. Holland), were studied during 1968–70. The lake with an average depth of 11 4 m (maximum depth 31 m) and area of 2·65 × 106m2is one of the deepest and biggest in the Loosdrecht‐lakes area. The lake is 125 cm below sea level, and underground water‐movements play an important part in the lake's hydrology.The lake exhibits thermal stratification on warm and calm days; since the lake surroundings are open and flat, wind and nocturnal cooling destroy such a stratification. There is continuous circulation from autumn through spring. The O2 saturation (%) in the upper water varies from 70 to 120%. Bottom waters were never anaerobic (lowest values 10% O2saturation).CI−(2·8m‐equiv.) and HCO−3(1·9 m‐equiv.) were the dominant anions just as Ca++(2·77 m‐equiv.) and Na+(2·5 m‐equiv.) formed the main cations. Chlorides have increased 2·5‐fold in the 40 years as a geochemical consequence of deepening. The surface drainage has minor influence on lake's water chemistry. Part‐P (10–140 μg/I) and PO4‐P (2–40 μg/1) recorded maximum and minimum respectively, and NO3‐N (0·05–1·15 mg/1) its minimum, during Microcystis abundance in August 1968. The SiO2‐Si decreased from February 1969 (400 μg/l) to June 1969 asDiatotna elongatumincreased. The Si‐decrease to<30% of the 1932 values is due to removal of Si‐rich clay and silt, due to sand digging.Chlorococcales were the important lake algae. Desmids were poor. Microcystis dominated as a rule from July‐September, achieving from 15 to 31 colonies/ml. Poor light transmission rather than nutrients limits plankton growth as also the primary production in the lake. Copepoda were the dominant zooplankton. Bosmina coregoni recorded between 2 and 44 individuaIs/1 in summer 1968 and was the main cladoceran. The average primary production during summers of 1969 and 1970 was 380 and 497 mg C m−2day−1respectively. Light limited production below 1 m—1 % light in 1969 and 10% in 1970 penetrated down to 4 m. About 70% of the production took place in the upper 2 m. Calculation of production according to theoretical models under‐estimated the observed values by 12% because Z0·5Iklay much above (0·8–2·8 m) the expected value of 3·5 m. It is suggested that in turbid lakes like Wijde Blikin situin