Microcystins LR and RR in two lakes were identified and their concentr
ations were determined. Water blooms containing Microcystis aeruginosa
, M. viridis and M. weisenvergii covered the surface of both lakes. M.
aeruginosa was the most prevalent toxic cyanobacterium detected. Micr
ocystin was identified by the mass spectra of the oxidation product, 2
-methyl-3-methoxy-4 phenyl butyric acid. Microcystin RR was detected a
t a concentration of 8.8 mug/l of lake A water collected in June, 1992
, whereas that of microcystin LR was 1.5 mug/l. Microcystin RR in lake
A water was detected during the October (the warm season) but microcy
stin LR was not detected in the period from July to October. The conce
ntration of microcystin RR is associated with that of chlorophyll-a in
water blooms.