Detection of circulating tumor cells (CTC's) in human blood and lymph systems has the potential to aid clinical decision making in the treatment of cancer. The presence of CTC's may signify the onset of metastasis, indicate relapse, or may be used to monitor disease progression. A photoacoustic flowmetry system was designed and tested for detecting circulating melanoma cells (CMC's) by exploiting the broadband absorption spectrum of melanin within CMC's. The device was tested on cultured melanoma cells in saline suspension and in a Stage IV melanoma patient. The device showed a detection threshold of a single melanotic melanoma cell from culture. Transient photoacoustic events were detected in a sample derived from a Stage IV melanoma patient that corresponded to particles passing through the laser beam path, indicating the presence of single melanoma cells in the human circulatory system.