Notoacmea petterdi occurs only on vertical surfaces at the highest levels on the shore. On a given surface, density of limpets decreases with increasing height. Mean and maximum sizes of limpets increase with increasing height, due to faster rates of growth at higher levels where densities are lower. Rates of mortality are also negatively correlated with height on the shore, and are therefore generally less at higher levels. Both adult and juvenile limpets exhibit well-developed homing behaviour. There is no evidence of seasonal migrations, density-dependent migrations, nor gradual upward migrations as limpets grow older. Hypotheses are formed on the basis of these preliminary observations and measurements. Results of subsequent experimental manipulations of densities support the hypothesis that density-dependent mortality, due to intraspecific competition, plays a major role in regulating population densities of this limpet. It is also suggested that the pattern of settlement of juvenile limpets is the major factor that determines the limits to the vertical distribution of this species.