The existing literature on phototropism in marine macroalgae is reviewed. A phototropic response is a sequence of three different processes: reception of the directional light signal; transformation of the signal to a physiological response (transduction); and the production of directional growth response. The only available information about the photoreception of directional light comes from action spectra investigations. Al1 the seaweeds thus far examined show a common pattern, with the maximum response occurring between 410-497 nm. A blue-light sensitive photoreceptor has therefore been postulated, and this photoreceptor is assumed to be a flavoprotein or a similar flavin molecule. The nature of the messenger, the chemical responsible for triggering the growth response, is unknown in marine algae. Evidence for the Cholodny- Went theory of phototropic movement, which postulates movement by lateral auxin transport, is reviewed together with studies of the presence of higher plant growth regulators in seaweeds. Some evidence points to cAMP as a mediator through changes in membrane potential, and the effect of Ca2+ in the reversion of positive phototropism has been recently demostrated. The phosphorylation of an unknown 100-kDa protein has also been associated with the transduction of the directional signal. The growth response is produced by bulging, i.e., a change in the growth centre. This response is the result of changes in the conformation of the cell wall mediated by plasmatic microtubules. The similarity between this process and other morphogenetic processes in seaweeds is discussed. The phototropic response in seaweeds appears to be ecologically effective, provided that the light field in the sea is highly directional, the proportion of blue light increases with depth and growing towards the light confers an advantage to upright thalli, while growing away from light is used by rhizoidal and attachment systems.