Abstract.To solve the problems of uniformization and moduli for Riemann surfaces, covering spaces and covering mappings must be constructed, and the parameters on which they depend must be determined. When the Riemann surface is a punctured torus this can be done quite explicitly in several ways. The covering mappings are related by an ordinary differential equation, the Lamé equation. There is a constant in this equation which is called the "accessory parameter". In this paper we study the behavior of this accessory parameter in two ways. First, we use Hill's method to obtain implicit relationships among the moduli of the different uniformizations and the accessory parameter. We prove that the accessory parameter is not suitable as a modulus-even locally. Then we use a computer and numerical techniques to determine more explicitly the character of the singularities of the accessory parameter. 0. Introduction. In the classical theory of uniformization of tori there are two distinct models for the space of moduli: the complex analytic t (the period ratio) in the upper half plane and the real analytic trace parameters of Fricke for the Fuchsian uniformization of a punctured torus. Our original goal was to find an explicit relation between these models of Teichmüller space. They are related via an ordinary differential equation which is completely determined except for an unknown constant, called the "accessory parameter". Explicit computation of the accessory parameter then would give the desired relationship and also seemed to give another model for the moduli space. Indeed, in a modern version [3], the accessory parameter which arises in the quasi-Fuchsian uniformization of the punctured torus is a complex analytic parameter for the moduli space. Accessory parameters are interesting in their own right. There is an extensive discussion of accessory parameters in the classical literature; however, little of a concrete nature was known. We hope that what we have done here sheds some light on the general problem of accessory parameters.In this paper, we prove that the accessory parameter is not even a local parameter for Teichmüller space and we give qualitative information about it;