Over 70 crude plant extracts, primarily from Kazakhstan, were screened against the freshwater phytoplanktons Oscillatoria perornata and Selenastrum capricornutum. Extracts from Limonium myrianthum and Inula helenium both demonstrated selective inhibition against the odorproducing cyanobacterium O. perornata compared to activity against the green alga S. capricornutum. Bioassay-guided fractionation of the L. myrianthum dichloromethane extract resulted in the identification of nepodin, torachrysone, chrysophanol, and physcion. Nepodin demonstrated the desired selective inhibition with lowest-complete-inhibition concentration values of 100 µg/mL and >100 µg/mL towards O. perornata and S. capricornutum, respectively. Similarly, chrysophanol also demonstrated the desired selective inhibition with lowest-completeinhibition concentration values of 10 µg/mL and >100 µg/mL, respectively. Bioassay-guided fractionation of the I. helenium hexane extract resulted in the isolation of alantolactone, isoalantolactone, and 11αH,13-dihydroisoalantolactone. Activities of these three isolated constituents as well as those of synthetic isomers are reported.