Feeding by the nematode X. diversicaudatum caused a progressive increase in the DNA content and size of strawberry nuclei. After four days feeding, nuclei had DNA values intermediate between 8C and 16C and had increased in size from a mean of 17 micron2 for control root tips to 49 micron2. Multinucleate cells were present after two and four days feeding. There were no ultrastructural differences in the composition of nuclei from control and parasitized root tips, but strawberry nuclei consisted mainly of dispersed chromatin whereas ryegrass nuclei contained a large proportion of condensed chromatin.