  We report on Australia Telescope Compact Array observations of the ~10^5 yr
old pulsar PSR B0906-49. In an image containing only off-pulse emission, we
find a weak, slightly extended source coincident with the pulsar's position,
which we argue is best interpreted as a pulsar wind nebula (PWN). A trail of
emission extending behind the pulsar aligns with the major axis of the PWN, and
implies that the pulsar is moving north-west with projected velocity ~60 km/s,
consistent with its scintillation speed. The consequent density we infer for
the pulsar's environment is >2 cm^(-3), so that the PWN around PSR B0906-49 is
confined mainly by the high density of its surroundings rather than by the
pulsar's velocity. Other properties of the system such as the PWN's low
luminosity and apparent steep spectrum, and the pulsar's large characteristic
age, lead us to suggest that this nebula is substantially different from other
radio PWNe, and may represent a transition between young pulsars with prominent
radio PWNe and older pulsars for which no radio PWN has been detected. We
recommend that further searches for radio PWNe should be made as here: at low
frequencies and with the pulsed emission subtracted.
