One of the main results announced in Part I (Compositio Math. 139 (2003), 197–237) is proved. The main technique consists in developing a coordinate free version of some of the theory of Serre operators on differential modular forms introduced by Barcau.

The purpose of this article is to develop the theory of differential modular forms introduced by A. Buium. The main points are the construction of many isogeny covariant differential modular forms and some auxiliary (nonisogeny covariant) forms and an extension of the ‘classical theory’ of Serre differential operators on modular forms to a theory of ‘δ-Serre differential operators’ on differential modular forms. As an application, we shall give a geometric realization of the space of elliptic curves up to isogeny.

An interesting theory arises when the classical theory of modular forms is expanded to include differential analogs of modular forms. One of the main motivations for expanding the theory of modular forms is the existence of differential modular forms with a remarkable property, called isogeny covariance, that classical modular forms cannot possess. Among isogeny covariant differential modular forms there exists a particular modular form that plays a central role in the theory. The main result presented in the article will be the explicit computation modulo p of this fundamental isogeny covariant differential modular form.