It has been shown that a virulent phageQ&bgr; can be cultured continuously in a cellstat which was developed to culture a bacteriophagefdfor the study of molecular evolution. A cloned disadvantageous mutant was cultured to obtain a biological relaxation curve to selection/mutation equilibrium. A new technique for scraping off the wall growth by air bubbles is introduced. The scraper worked well in short‐time cultivation ofQ&bgr;. It has various advantages of a closed system.