You do not need a job before entering the u.s., you can work multiple jobs at the same time, and you can change employers at any time. There are a number of different exchange visitor (j) visas that allow for temporary entry to work in the united states Does not have a traditional working holiday visa program like other countries such as australia or new zealand, there are alternative programs that provide similar opportunities. Can i work in the us with an australian working holiday visa No, the australian working holiday visa (subclass 417) does not allow you to work in the us.
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