Plants from the genus Centaurea (C.) (Family: Asteraceae alt. Compositae), widely distributed in Asia, Europe and North America, have traditionally been used in the treatment of various ailments. As a part of our on-going studies on the plants from the genus C. for their phytochemistry and biological activities, extracts of the seeds of Turkish endemic C. species, C. bornmuelleri and C. huber-morathii, were tested for their cytotoxicity towards the CaCo2 colon cancer cell line as well as for the toxicity towards the brine shrimps, using the MTT and the brine shrimp lethality assays, respectively. Among the extracts, the MeOH extract of these plants showed significant toxicity towards the brine shrimps ($LD_{50}=55.2{ imes}10^{-2};and;42.4{ imes}10^{-2}mg/ml$, respectively). The MeOH extract of both C. species also inhibited the growth of CaCo2 colon cancer cells in the MTT assay ($IC_{50}$=29.9 and 33.0 g/ml, respectively). As the most prominent activities in both assays were observed with the MeOH extracts, it can be assumed that the compound(s) responsible for these activities are polar in nature.