  We have investigated how the incidence of star-forming galaxies varies with
the environment using a volume-limited sample of nearby galaxy spectra
extracted from the 2dF Galaxy Redshift Survey. The environment is characterized
by the local number density of galaxies, corrected for sample incompleteness
and border effects. Using the equivalent widths of [O II]$\lambda$3727 and
H$\delta$ we discriminate the star-forming galaxies in two classes, starbursts
and ordinary star-forming galaxies, and evaluate their properties as a function
of the local density. We show that the fraction of ordinary star-forming
galaxies decreases regularly with increasing density, indicating that star
formation is sensitive to the environment for all range of densities present in
the sample. The fraction of starbursts is approximately independent of the
density and, consequently, the fraction of starbursts relative to star-forming
galaxies increases with local density. This suggests that a mechanism that acts
everywhere, like tidal interactions, is the responsible for triggering
starbursts. We also show that while the mean EW(H$\alpha$) of ordinary
star-forming galaxies is progressively reduced as density increases, for
starbursts it suffers a strong decrease at densities corresponding to scales of
$\rho^{-1/3} \sim 2.5 ~h^{-1}$ Mpc. A visual inspection of starburst images in
the {\it Digital Sky Survey} reveals that this corresponds to a change in the
morphology of starburst galaxies, from mainly apparently normal spirals and
galaxies with apparently compact morphologies at low densities, to tidally
distorted spirals at high densities.
