Sulphur dioxide lesions on alfalfa are of 2 types, acute and chlorotic. The former shows characteristic bleached interveinal and marginal areas in otherwise normally appearing leaves; the latter exhibits a more or less yellowed and mottled appearance and is similar to chlorosis as produced by a number of other causes. The reduction in yield of alfalfa subjected to a single SO2 fumigation of one crop is in direct proportion to the %age of leaf area destroyed. The stage of growth at which the fumigation occurs does not appear to influence the result, at least within the range of 25-75% of the total growth period of the plant. If a crop is fumigated more than once, assuming at least a week to elapse between fumigations each of which produces the same %age of leaf destruction, the reduction of the yield is also in proportion to the number of fumigations. The reduction in yield is not a linear function of the number of leaves marked, indicating that the uninjured portions of the leaves continue to function in spite of bleached areas. The yield-leaf-destruction curves all closely approach 100% yield at 0-per cent. leaf destruction, indicating that the gas does not reduce the yield unless it produces visible effects. The reductions of yield caused by SO2 fumigation can be closely duplicated by clipping from normal plants an amount of leaf tissue equivalent in area to that destroyed by fumigation. Like acute markings, chlorotic SO2 markings appear to lower the yield in direct proportion to the %age of leaf tissue visibly affected. A severe defoliation treatment, either by fumigation or by clipping, which lowers the yield of the treated crop extensively, also reduces the yield of the subsequent untreated crop appreciably, and may even be felt slightly in the 2nd subsequent untreated crop. The subsequent growth of new leaves, and the elongation of the stems following a partial defoliation, proceed at the same rate, regardless of whether the leaf destruction was accomplished by SO2 fumigation or by clipping.