We study the self-assembly of amphiphile molecules in water as a function of temperature and concentration in Monte Carlo simulations on a two-dimensional lattice, in which each water molecule occupies a single site, and each amphiphile occupies three sites. We determine the free amphiphile concentration and the aggregate-size distribution P( n) as functions of total surfactant concentration for different temperatures. The degree of micellar organization is characterized by Delta=P(n(max))-P(n(min)), where n(min) and n(max)>n(min) are the aggregate sizes at which P exhibits its local minimum and maximum. We show that Delta vanishes continuously as we increase the temperature, and find the exponent associated with this micellar to nonmicellar transition.