The role of microtubules in the secretory processes in the tegument of adult trematode Fasciola hepatica L. is studied by estimating the effects of colchicine, a substance known to disrupt microtubules, on the number of T2 vesicles. Tissue slices of Ihe worm are incubated in Hedon-Fleig medium with or without 5 x 10'4M colchicine. The dynamics of the colchicine-provoked secretory block is examined by morphometry on samples processed for electron microscopy. T2 vesicles are estimated as a total number or separately within three levels (apical, sub-apical and central) of the distal tegument. The secretory block is demonstrated as reduction in the total number of T2 vesicles. The separate counting within three levels of the distal tegument demonstrates in control samples a trend of sub-apical condensation of T2 vesicles. This pattern of T2 distribution remains unchanged in colchicine-treated samples in spite of the reduction of the mean T2 counts within each of the levels examined. The data illustrate the role of microtubules in both the tegumental transport of secretory vesicles and the stratification of the organelles within the tegument.