Auxiliary beta subunits play a major role in defining the functional properties of large-conductance, Ca2+-dependent BK-type K+ channels. In particular, both the beta1 and beta2 subunits produce strong shifts in the voltage dependence of channel activation at a given Ca2+. Beta subunits are thought to coassemble with alpha subunits in a 1:1 stoichiometry, such that a full ion channel complex may contain up to four beta subunits per channel. However, previous results raise the possibility that ion channels with less than a full complement of beta subunits may also occur. The functional consequence of channels with differing stoichiometries remains unknown. Here, using expression of alpha and beta subunits in Xenopus oocytes, we show explicitly that functional BK channels can arise with less than four beta subunits. Furthermore, the results show that, for both the beta1 and beta2 subunits, each individual beta subunit produces an essentially identical, incremental effect on the voltage dependence of gating. For channels arising from alpha + beta2 subunits, the number of beta2 subunits per channel also has a substantial impact on properties of steady-state inactivation and recovery from inactivation. Thus, the stoichiometry of alpha:beta subunit assembly can play a major functional role in defining the apparent Ca2+ dependence of activation of BK channels and in influencing the availability of BK channels for activation.