  We present results from integral field optical spectroscopy with the Potsdam
Multi-Aperture Spectrograph of the Herbig-Haro (HH) object HH 204, with a
spatial sampling of 1 x 1 arcsec^2. We have obtained maps of different emission
lines, physical conditions and ionic abundances from collisionally excited
lines. The ionization structure of the object indicates that the head of the
bow shock is optically thick and has developed a trapped ionization front. The
density at the head is at least five times larger than in the background
ionized gas. We discover a narrow arc of high T_e([N II]) values delineating
the southeast edge of the head. The temperature in this zone is about 1,000 K
higher than in the rest of the field and should correspond to a shock-heated
zone at the leading working surface of the gas flow. This is the first time
this kind of feature is observed in a photoionized HH object. We find that the
O^+ and O abundance maps show anomalous values at separate areas of the bow
shock probably due to: a) overestimation of the collisional de-excitation
effects of the [O II] lines in the compressed gas at the head of the bow shock,
and b) the use of a too high T_e([N II]) at the area of the leading working
surface of the flow.
