Most of the carbon fixed in photosynthesis by Xanthoria aureola (X. parietina var. aureola) initially accumulates in sugar alcohols and relatively little is incorporated into insoluble compounds at first. Two kinds of sugar alcohol are involved—mannitol and a pentitol. During the early stages of photosynthesis, much more carbon accumulates in the pentitol than in mannitol. Within 9–15 minutes of exposure to NaH14CO3 in the hght, the level of 14C in metabolic intermediates presumably common to both symbionts (PGA, hexose monophosphates, organic acids) became more or less steady and remained so for several hours. It was concluded from this that it is likely that the flow of carbon from alga to fungus is relatively rapid and efficient, and achieved a steady rate within 15 minutes of the start of the experiment.