Lower Bostraze And Leswidden SSSI is a biological Site of Special Scientific Interest, located on the Penwith Peninsula, Cornwall, England, a little to the east of St Just, approximately 8 km west of Penzance.

This species is rare in a British context (it is known from only two other locations in the UK: St Austell Clay Pits and Tregonning Hill, both within Cornwall).

At Lower Bostraze and Leswidden, Western Rustwort is generally found growing on micaceous or clay waste substrates which are flat or gently sloping.

It appears to be a pioneer species, the largest populations being found on surfaces showing the early stages of colonisation by other bryophytes and by vascular plants.

[1] The conservation charity Plantlife designated the St Just Moors, part of this SSSI, an Important Plant Area for the occurrence of Western Rustwort and the green algae desmids.