All species of non-filamentous algae found in the water and on the sediment of Highland Water (New Forest) [England] contributed to the diet of larval lampreys (L. planeri). The algae formed only 0.01 and 0.3% of the total volume of the gut contents in the winter and summer, respectively, although in a more productive stream their contribution was greater (1.5%). Between the onset of metamorphosis in Aug. and the period just prior to spawning in April, the amount of lipid in L. planeri fell from 14.6-8.9% of the wet wt. During metamorphosis in L. fluviatilis the lipid content decreased from 14.1% in Sept. to 7.9% in the following March. The progressive decrease in percentage lipid in metamorphosing L. planeri and L. fluviatilis was accompanied by an increase in percentage water. In L. fluviatilis the absolute lipid levels declined throughout metamorphosis whereas the absolute water content rose initially and then showed a marked declined during Nov. The combined loss of lipid and water at this time resulted in a marked decline in the animal's wet wt. Lipid levels in adult L. fluviatilis fell from approximately 18% in migrants caught in the estuary during Oct. to 3% in spent animals taken from tributary rivers in late March. Over the migratory period the amount of neutral lipid in the lipid fraction decreased from approximately 80 to 14-22%. The migrating males contained a larger amount of lipid in their liver than females whereas the reverse situation was found in the gonads.