Bamboo fibre underwear company Step One is preparing to spend about $12 million on an entry into the US market as it readies to list on the ASX in November in a public float that values the group at $284 million.

Step One said in its prospectus it has captured 0.8 per cent of the British male underwear market in the past 18 months, and the United Kingdom now makes up 34 per cent of total sales, with 66 per cent coming from its home market of Australia.