A photographic study of the wakes of slugs rising in tubes of 19 mm and 52 mm internal diameter is presented. The dependence of the flow pattern in the wake upon the Reynolds number of the rising slug, R, is established for different slug lengths. Values of R covered in this study are in the range 25 to 1.3 × 10⁴. For low values of R the flow pattern in the wake is laminar and axisymmetric and values of wake length and wake volume could be determined from the photographs: these values were correlated with the other variables in the system by means of dimensional analysis.