This paper presents a parameterization of the fraction of aerosol particles activated to form cloud droplets in a parcel of air rising adiabatically. The study applies to aerosols of a single lognormal size distribution with uniform chemical composition and capable of serving as cloud condensation nuclei. The parameterization is based on analysis of the rate equations for the parcel supersaturation and aerosol activation process. The analysis leads to the identification of only four dimensionless parameters on which the fraction of activation strongly depends. Using results of detailed numerical simulations by a size-resolving Lagrangian parcel model, errors due to simplifying assumptions used in the analysis were largely eliminated by employing adjustment coefficients. For a wide range of governing parameters (e.g., particle radius, standard deviation, updraft velocity, etc.), differences between the parametric equations and the numerical model results are less than 10% for most conditions and less than 25% for some extreme but realistic conditions. This new parameterization is significantly more accurate and successful in representing the fraction of activation in terms of governing parameters than any known parameterization. ¿ 1998 American Geophysical Union