Flow patterns that occur in a rod bundle with forced upward flow of boiling water have been photographed. The experimental rod bundle consisted of four vertical 0.25-in.-diam × 24.0-in. long rods arranged in a single row with spring collar supports. The tested conditions were: pressure—400 to 2000 psia, mass velocity—250 000 to 3 000 000 lb/h-ft2, and heat flux (uniform)—up to 1 600 000 Btu/h·ft2. The observed two-phase flow patterns were bubble flow, froth flow, slug flow, and annular flow. These flow patterns were mapped at constant pressure on plots of mass velocity versus flowing quality.