Temporal and spatial expression of embryonic alkaline phosphatase (ALP) activity in the oligochaeteTubifex hattaiwas examined by means of histochemical staining. During the course of development up to 7 days (at 22°C), ALP activity emerged in three distinct structures in the following order: dorsal surface linear structures running along the midline, nephridium-like structures located in the mesodermal territory of segments 7 and 8, and ventral vessels located in the space between ventral ganglia and the endoderm. Stage-by-stage observation showed that the dorsal linear structures originate as a bilateral pair of small cell clusters that appear at the anterior end of the early gastrula embryo. Likewise, each nephridium-like structure was traced back to a single ALP-positive cell that emerges at the ventrolateral region in each of segments 7 and 8 at the late gastrula stage. Ventral vessels became ALP-positive, first in segments 5–13 and then in more posterior segments. No ALP activity was detected in the endoderm during embryogenesis.