Twenty-four men and women were tested using the crawl, back (inverted crawl), breast, butterfly, and overarm side strokes. The breath was held during swimming and the amount of energy used was computed from the O2 debt using the Douglas-Hal-dane method. The strokes arranged in order of increasing energy cost were as follows: crawl, back, breast and side. The butterfly stroke is the least economical of the 5 strokes under 2.5 ft./sec. Above this speed it is more efficient than the side stroke; at 3 ft./sec. it is more economical than the breast stroke. Swimming at speeds over 2 ft./sec. should be considered vigorous exercise. Inexperienced swim-mers may raise their metabolism 10 times over basal at speeds less than 3 ft./sec. A good back stroke swimmer at a speed of 5.25 ft./sec. expended 150 cal./min. (14. H. P.). No sex difference in the cost of swimming was observed.