We have developed an instrument based on a flow cytometer platform that is capable of tracking individual, suspended cells over extended time periods. The instrument repeatedly moves in a capillary the same volume segment of fluid containing tens to hundreds of suspended cells through the focal point of a laser. Individual cells are then tracked based on the timing of when they cross the laser, and cell properties are measured as in a conventional flow cytometer. Because cells are repeatedly measured the single-cell rates of change can be determined. The developed instrumentation was applied to measure the variability of ABC transporter activity in a population of human cancer cells and the temperature dependence of constitutively expressed Gfp in yeast. A wide range of transport rates can be observed in the cancer cell population while the single-cell Gfp fluorescence in yeast shows pronounced oscillations in response to temperature shifts. These observations are not detectable at the population level. Therefore, such measurements are useful for investigating cell function as they reveal how variable properties of single cells change over time.