Atomic indium, produced by the pyrolysis of trimethylindium (TMIn) during metalorganic chemical vapor deposition, has been directly observed using resonant fluorescence spectroscopy. The indium fluorescence signal is linear with TMIn flow rate at a susceptor temperature of 500 °C above a small background threshold level. The threshold for the observation of an indium signal is 325 °C. Addition of trimethylantimony (TMSb) at concentrations less than those required for normal InSb epitaxial growth results in quenching of the indium signal.