This study was conducted to test whether the ethanolic extract of the plant Cyperus kyllingia can exert antimitotic effects in actively dividing onion root tip cells, and toxic effects on alive brine shrimp. Onion bulbs and brine shrimps were exposed to solutions of C. kyllingia ethanolic extract at various concentrations for 24 h. Results showed that the extract can block mitosis in onion cells by causing spindle and chromosome damage, which results to mitotic anomalies. In effect, it induces pyknosis and cell death. Moreover, the effects on mitotic index, percentage of C-mitosis, and percentage of other mitotic anomalies are dose-dependent. The test extract can cause dose-dependent lethality in brine shrimp with an estimated LC₅₀ value of 13.47 ppm. The findings indicate that the test plant contains antimitotic and cytotoxic components that may be studied further in animal test systems as possible models of anticancer drugs.