The first results regarding the crystallization kinetics of the pure semisynthetic beta -lactam antibiotic (SSA) ampicillin in water are presented, The experimentally obtained results at pH = 5 and T = 298 K are described well by the population-based crystallization model presented in this paper. The nucleation rate J (# m(-3) s(-1)) is related to the supersaturation ratio of ampicillin S by J = 2 x 10(7) exp[-3.383/(ln(S))(2)]. The surface energy gamma of creating a nucleus of ampicillin molecules therefore equals 5.83 mJ m(-2). The growth rate G (in s-1) is determined with a newly developed adapted single-crystal growth rate analysis method and is related to the supersaturation ratio of ampicillin S by G = 4.57 x 10(-8)(S -1)(1.72).