  In this note, we consider an l-isogeny descent on a pair of elliptic curves
over Q. We assume that l > 3 is a prime. The main result expresses the relevant
Selmer groups as kernels of simple explicit maps between finite- dimensional
F_l-vector spaces defined in terms of the splitting fields of the kernels of
the two isogenies. We give examples of proving the l-part of the Birch and
Swinnerton-Dyer conjectural formula for certain curves of small conductor.
