Microgametes of Eimeria tenella were observed in cecal tissue from infected chicks killed six days after inoculation with oocysts. Development from the microgamont stage to maturity was followed. We observed three commonly found, but distinctly different, stages in developing microgametes while they were still associated with microgamonts. In the first, only the flagella could be seen protruding through the enveloping microgamont layer. In the second, both the bodies and flagella could be seen; the flagella had increased in size, and the bodies appeared bulbous. In the third, nearly mature microgametes were randomly arranged around the microgamont, and the manner of flagellar attachment to the bodies near the apical conjunction of the perforatorium was seen. A free microgamete and a macrogamete with one penetrating and two adhering microgametes were found. On flagellum and most of the body of the penetrating microgamete had entered the macrogamete. All micorgametes seen had two flagella.