The heart of Schoof's algorithm for computing the cardinality m of an
elliptic curve over a finite field is the computation of m modulo
small primes l. Elkies and Atkin have designed practical
improvements to the basic algorithm, that make use of "good" primes
l. We show how to use powers of good primes in an efficient way.
This is done by computing isogenies between curves over the ground
field. A new structure appears, called "isogeny cycle". We
investigate some properties of this structure.