In an evaporating trickling film steam bubbles can form by adequate fluid overheating. These occur in quick succession in suitable pittings in the heating surface, are torn away from the surface by the current, swim in or on the trickling film, getting larger until they burst. They blend the film considerably. Heat transfer to the latter is improved by the steam bubbles. In very slow motion the film shows the behaviour of the bubbles with calm and rippled current. The fluid flows on the outside of a heated test tube. Fluid and heating area loads are varied. The effect of a rough tube surface is also shown. With suitably high heating area load a continuous bubble layer occurs, which disperses with more intense overheating.