Mountain biking is popular in many natural areas, but remains controversial, at least in part, due to divergent views about its environmental impacts. In contrast to research assessing the risk of activities such as horse riding and hiking spreading weed seed, similar data for mountain bikes does not appear to be available in the academic literature. To start to address this gap, we present the results of a preliminary experiment comparing seed attachment to a horse and a mountain bike in dry conditions along 20 1 m by 50 m transects through areas where weeds are seeding. In total, seed from more than 12 species were found on the horse and more than 10 species on the bike. Per transect, a greater diversity of seed attached to the horse (6 vs 4 morphtaxa) than the bike, but they had similar numbers of seed (Average=22). When seed composition per transect was compared using ordinations, there were clear differences with more seed from non-native grasses such as Chloris virgate and Chloris gayana, the native grass Dicantheum scericeum, and the non-native herb Vicia sativa on the bike, while on the horse there tended to be more seed from the grass Poa queenslandica (native) and the Axonopus fissifolius (non-native). This pilot study demonstrates how mountain bikes can carry seed from a diversity of weeds in Australia. More extensive testing will better quantify the types and amount of seed that could be dispersed, as well as test the effect of factors such as weather conditions, timing and location of rides on seed dispersal by bikes. In the interim, recommendations for bikes to be regularly cleaned, including between rides in areas of high conservation value are likely to help reduce the risk of mountain biking spreading weed seed.