The characters of cell differentiation and cell apoptosis of Dictyostelium discoideum , especially slug, were studied by Acridine orange and Hoechst 33342 staining, two vital fluorescence dyes, with the help of fluorescence microscopy. The results show that different degree apoptosis of the pre-stalk cells during development were referred to the colors, blue-green, green, orange, and finally colorless. In the later of slug stage, a channel-like structure were observed. These data indicate that slug is the initiation phase of cell differentiation and recessionary in Dictyostelium discoideum. Furthermore, the morphological characters change greatly during the slug stage. The pre-stalk cells have already initiated apoptosis since the early stage of slug period. However, their apoptosis is a gradual process.