A phototrigger system is developed as a part of a video microscope mounting an ultrahigh-speed video camera capable of image capturing at frame rates as high as 1x10(6) framess. The extremely high frame rate is achieved by implementing in situ image storage. A distinguished feature of the camera is the on-chip overwriting mechanism that allows to keep in storage the latest image sequence of 103 frames; the old signals are continuously drained out of the storage. The trigger system is designed to synchronize recording operations with an occurrence of a target event within the limited image capturing duration. The target event is detected through a sudden change in the output of a sensor mounted to an optical port of the microscope. To reduce noise contribution, a two-sensor architecture is implemented. One sensor detects the target event while the one produces a reference signal used for noise reduction. Both sensors are connected to the same optical port by using a specially designed beam splitting unit. To provide high sensitivity, avalanche photodiodes are used as photoelements. System evaluation shows that its sensitivity is high and response time is less than 3 mus. This is sufficiently fast for high-speed video-microscopy observations at 1x10(6) frames/s when using a video camera with a storage of 103 frames. As an example, the system was used in a microscopic observation of a soap film collapse.