  Temperature dependence of the magnetic penetration depth, $\lambda$(T), was
measured in the Meissner state in single crystals B$_{1-x}$K$_{x}$BiO$_{3}$
(x=0.37) using tunnel diode resonator technique. At low temperatures, $0.013
\le T/T_c \le 0.4$, $\lambda$(T) is exponentially flat, which provides a strong
evidence for conventional s-wave BCS behavior. Numerical analysis of the data
rules out the possibility of a gap with nodes.
