Three underground streams were dye traced as many as twenty-four times, at various flows between the extremes of flood and drought. This systematic study, the first of its kind to our knowledge, has shown that: 1. Travel time (the time between input of dye at the swallet and its first arrival at the resurgence) is inversely proportional to mean resurgence output over the same period. This is characteristic of simple phreatic streams, which should be distinguishable using graphic analysis from vadose and complex phreatic streams. 2. Rhodamine WT dye, the most stable of the common fluorescent dyes, is progressively lost, to a significant and unpredictable extent, in transit from swallet to resurgence. Succesful tracing therefore requires more dye at low flows than at high flows.