IN a previous publication1, we suggested that the classical ‘squeezing from behind’ mechanism for protoplasmic streaming is inadequate to explain the actual movements observed in slime-mould plasmodia. As a result of a recent change in culture conditions, remarkably simple and direct evidence to substantiate this suggestion has appeared. Fig. 1 is a photograph of a plasmodium of Physarum polycephalum after two days growth at 22° C. in the dark on a particular growth medium. This medium contains, per litre, 20 gm. agar; 5 gm. ‘Bacto-tryptone’; 5 gm. dextrose; 0.75 gm. ‘Bacto-yeast’ extract; 5 mgm. haemin; 0.8 gm. potassium dihydrogen phosphate; 0.2 gm. disodium hydrogen phosphate; 100 mgm. calcium chloride; 100 mgm. magnesium sulphate; 200mgm. citric acid; 10 mgm. ferrous chloride; 14 mgm. manganese chloride and 6 mgm. zinc sulphate, adjusted to pH 6.2 and autoclaved for 30 min. at 15 lb./sq. in. (This is modified from the formula kindly supplied to us by Dr. H. P. Eusch, University of Wisconsin.)