Resonantly enhanced three‐photon ionization spectroscopy is employed for the detection of ground‐state atomic hydrogen and deuterium. Concentrations as low as 4×109atoms/cm3in the presence of 1017atoms/cm3of buffer gas are detected with an implicit time resolution of less than 10 nsec. A useful dynamic range of at least 4×104is demonstrated and saturation of the ionization in a 2×10−4‐cm3focal volume is observed. The presence of H can easily be distinguished from that of D and the three‐dimensional spatial distribution of H or D can be determined.