Mode partition noise is unavoidable with semiconductor lasers, even when the laser is oscillating in a nearly single longitudinal mode.1-4 This noise shows up in the form of power dropouts in the main mode, and power increases in the side modes. Such power dropouts were first observed by Linke et al.5 Main-mode power dropouts can affect the performance of a coherent optical communication system in the form of signal fading. Knowledge of the statistics of dropout pulse interarrival times, depths, and widths is important if one wants to estimate the error rate of the system. We have studied these statistics using commercial single-mode laser diodes.