We have previously demonstrated the expression of large-conductance, calcium-activated potassium (BK) channels in human glioma cells. In the present study, we characterized the calcium sensitivity of glioma BK channels in excised membrane patches. Channels in inside-out patches were activated at −60 mV by 2.1 × 10−6 M cytosolic Ca2+, were highly K+-selective, and had a slope conductance of ≈210 pS. We characterized the Ca2+ sensitivity of these channels in detail by isolating BK currents in outside-out patches with different free [Ca2+]i. The half-maximal voltage for channel activation, V0.5, of glioma BK currents in outside-out patches was +138 mV with 0 Ca2+/10 EGTA. V0.5 was shifted to +81 mV and −14 mV with free [Ca2+]i of 1.5 × 10−7 M and 2.1 × 10−6 M, respectively. These results suggest that glioma BK channels have a higher Ca2+ sensitivity than that described in many other human preparations. Data obtained from a cloned BK channel (hbr5) expressed in HEK cells support the conclusion that glioma BK channels have an unusually high sensitivity to calcium. In addition, the sensitivity of glioma BK channels to the BK inhibitor tetrandrine suggests the expression of BK channel auxiliary β-subunits by glioma cells. Expression of the auxiliary β-subunit of BK channels by glioma cells may relate to the high Ca2+ sensitivity of glioma BK channels. GLIA 38:281–291, 2002. © 2002 Wiley-Liss, Inc.