A method is described which allows isolation of macronuclear DNA from the rumen ciliate Entodinium caudatum. Ciliate cells were enriched from the total microbial population in the rumen of sheep with a single ciliate species present by sedimentation and size filtration. Ciliates were then lysed by homogenisation and the macronuclear component isolated on a Percoll density gradient. Macronuclei can be stored in guanidine. HCl until ready for DNA extraction. DNA which had been extracted from macronuclei fresh from the Percoll gradient, or following guanidine. HCl storage, is of sufficient quality to permit its use for PCR analysis.