The far-infrared spectrum of the ring-puckering vibration of bicyclo[3.1.0]hexan-3-one has been recorded and analyzed. Ten closely spaced bands with a band origin at 86.4 cm(-1) were observed. Kinetic energy expansions were calculated for this molecule and also for bicyclo[3.1.0]hexane, 6-oxabicyclo[3.1.0]hexane, 3-oxabicyclo[3.1.0]hexane, and 3,6-dioxabicyclo[3.1.0]hexane, and these expansions along with published data were utilized to calculate the asymmetric one-dimensional ring-puckering potential energy functions for each. For bicyclo[3.1.0]hexan-3-one the function is V (cm(-1)) (2.99 x 10(5))x(4) + (0.39 x 10(5))x(2) + (1.58 x 10(5))x(3), where x is the puckering coordinate in Angstrom. The experimental results were compared to molecular mechanics calculations in each case.