We consider the transport equation $\ppp_tu(x,t) + (H(x)\cdot \nabla u(x,t))
- p(x)u(x,t) = 0in\OOO \times (0,T)where\OOO \subset \R^n$ is a bounded
domain, and discuss two inverse problems which consist of determining a
vector-valued function H(x) or a real-valued function p(x) by initial
values and data on a subboundary of \OOO. Our results are conditional
stability of H"older type in a subdomain D provided that the outward normal
component of H(x) is positive on \pppD∩\ppp\OOO. The proofs are based
on a Carleman estimate where the weight function depends on H.