ABSTRACTRadioactive bicarbonate was pulse fed to bladesof Macrocystis pyrifera(L.) C. A. Ag. and the movement of the11C‐labelled photoassimilates was monitored in vivo using an externally mounted array of Geiger‐Müller detectors. Results of experiments conducted in August 1982 and February 1983 showed kinetic transport profiles composed of short pulses of11C (periods of two to three minutes and six to eight minutes) and a mass flow component travelling with a speed of 6–22 cm · h−1. The pulse‐like movement of11C‐photoassimilates, revealed for the first time in a kelp, may be driven by an energy‐assisted transport mechanism. Light microscopy revealed a putative symplastic transport pathway from the photo synthetic meristoderm to the medullary sieve cells in theM. pyriferablade. Of particular importance were the connections between the inner cortical cells and thin‐walled medullary sieve cells. Electron microscopy showed sieve plate pore diameters ranging between 35–60 nm in the cortex and ca. 40 nm in the end walls of the thin