Abstract
A mosaic of untwinned YBa(Formula presented)Cu(Formula presented)O(Formula presented) single crystals was studied by muon spin rotation. The flux-line lattice in these crystals was found to be pinned. This pinning then leads to disorder in the lattice. An estimate of the average relative breathing mode lattice displacement (Formula presented) was found to be about 0.8%, comparable to that found for twinned samples. The origin of the pinning in these crystals may be oxygen vacancies.