Colonisation by nymphs of eight mayfly species was monitored in a small stream using implanted natural substrate trays of different area during a seven day period.All species colonised the trays, but to various degrees. Downstream drift and directed or random crawling over the substrate contributed equally to colonisation of the implanted substrates.The area of the implanted substrate trays had profound effects on the diversity, density and size frequency distributions of the colonising assemblage. Density declined, but overall diversity and mean size of most mayfly species increased as tray size increased.The influence of implanted substrate tray area on the above parameters indicates potential problems in the use of colonisation studies to examine the composition and dynamics of lotic invertebrate assemblages. This is especially so given the lack of standardisation of colonisation samplers between such studies.