Seston (total particulate matter in lake water) was collected in traps and measured over at least one annual cycle in five Lake District lakes, of contrasted morphometry and productivity. The organic fraction was estimated in all lakes, total nitrogen and iodine in three, and in Blelham Tarn samples were analysed monthly for carbon, nitrogen, iodine, potassium and calcium. The depth represented by the annual catch of seston was compared with the depth of the present annual increment to the sediments, as determined from distribution of fallout 137Cs (present in the atmosphere since 1954). Conclusions reached were: (i) in shallow lakes resuspension and recirculation of once-deposited material, particularly at the time of the autumn breakdown in stratification, resulted in seston catches of twice the volume of the permanent increment to the sediments, whereas in the deep lakes there was no significant difference between the volume of seston trapped and the annual increment; (ii) results of analysis of autumn seston from Blelham Tarn agreed with the postulated origin in resuspended material from the bottom; (iii) though in all lakes the present annual accumulation rate exceeds the mean post-glacial rate, this excess is significantly greater in the more productive lakes--those in which evidence for some degree of eutrophication within the last two centuries is present in the sediment profiles; (iv) comparison of composition of seston and sediment shows a higher percentage loss of organic matter from seston in those lakes in which the hypolimnion remains oxygenated throughout the year; and (v) that these facts point to the survival into permanent sediment of organic matter from algal production in the two lakes which have become eutrophic--a situation which contrasts with that which has obtained in Lake District lakes through most of post-glacial time.