For the first time developmental stages of Theileria within the intestine and haemolymph of ticks were studied by electron microscopy. On the 1st-6th day post repletionem the intestine of the ticks contained humerous tages, which were very similar to microgamonts and microgametes of haemosporidia. Later, on the 14th-19th day p.r. ookinete-like stages ere found in the haemolymph of ticks. From these results it seemed likely that gamogony of Theileria takes place in the intestine of ticks. In more detail: The microgamont-like stages were spindle-shaped; they measured about 8-12 mu in length with a diameter of about. 08 mu in their middle region...