  We present photometry and spectra of the type IIP SN1999em in NGC 1637 from
several days after the outburst till day 642. An amount of $\approx 0.02
M_{\odot}$ of ejected $^{56}$Ni is inferred using the recovered bolometric
light curve . The H$\alpha$ and He I 10830 \AA lines at the nebular epoch show
that the distribution of the bulk of $^{56}$Ni can be represented approximately
by a sphere of $^{56}$Ni with a velocity of 1500 km s$^{-1}$, which is shifted
towards the far hemisphere by about 400 km s$^{-1}$. The fine structure of the
H$\alpha$ at the photospheric epoch reminiscent of the "Bochum event" in SN
1987A is analysed . The late time spectra show a dramatic transformation of the
[O I] 6300 \AA line profile between days 465 and 510, which we interpret as an
effect of dust condensation during this period. Late time photometry supports
the dust formation scenario after day 465. The [O I] line profile suggests that
the dust occupies a sphere with velocity $\approx 800$ km s$^{-1}$ and optical
depth $\gg10$. The plateau brightness and duration combined with the expansion
velocity suggest a presupernova radius of $120-150 R_{\odot}$, ejecta mass of
$10-11 M_{\odot}$ and explosion energy of $(0.5-1)\times10^{51}$ erg. The
ejecta mass combined with the neutron star and a conservative assumption about
mass loss implies the main sequence progenitor of $M_{\rm ms}\approx 12-14
M_{\odot}$. From the [O I] 6300,6364 \AA doublet luminosity we infer the oxygen
mass to be a factor four lower than in SN 1987A which is consistent with the
estimated SN 1999em progenitor mass . We note a "second-plateau" behaviour of
the light curve after the main plateau at the beginning of the radioactive
tail. This feature seems to be common to SNe IIP with low $^{56}$Ni mass.
