Organic petrology (incident light microscopy) of Middle Devonian inter-reef laminites and Devonian-Mississippian epicontinental black shales, Williston Basin, Canada, indicates that algal bloom episodes and consequential bacterial activity played a significant role in the accumulation of amorphous, bituminite III-rich organic microfacies. Corpohuminite-like algal akinete cells produced by filamentous algae during algal bloom periods are persistent maceral inclusions within the potential hydrocarbon source rock intervals. These cells (%R(o mean) range 0.24-0.90) are regarded as positive indicators of stressful palaeoenvironmental conditions. Unicellular Tasmanites and Leiosphaeridia marine alginite and variably degraded alginite remnants (＇＇ghosts＇＇) within the amorphous kerogen may be products of cell lysis, photo-oxidation and microbial alteration; these processes are characteristic of algal bloom periods. Minute (ca. 1 mu m) spheroidal and coccoidal bacteria-like macerals are dispersed throughout the bituminite III network, attesting to the importance of microbial activity within the water column and sediment during and after organic matter accumulation. Dispersed granules, laminations and replacement textures of micrinite-like macerals within bituminite III are interpreted as remnants of microbial alteration rather than a residual product of thermal maturation and hydrocarbon generation.