  We present the first ever calculation of cosmic microwave background CMB
anisotropy power spectra from semilocal cosmic strings, obtained via
simulations of a classical field theory. Semilocal strings are a type of
non-topological defect arising in some models of inflation motivated by
fundamental physics, and are thought to relax the constraints on the symmetry
breaking scale as compared to models with (topological) cosmic strings. We
derive constraints on the model parameters, including the string tension
parameter mu, from fits to cosmological data, and find that in this regard BPS
semilocal strings resemble global textures more than topological strings. The
observed microwave anisotropy at l = 10 is reproduced if Gmu = 5.3x10^{-6} (G
is Newton's constant). However as with other defects the spectral shape does
not match observations, and in models with inflationary perturbations plus
semilocal strings the 95% confidence level upper bound is Gmu<2.0x10^{-6} when
CMB data, Hubble Key Project and Big Bang Nucleosynthesis data are used (c.f.
Gmu<0.9x10^{-6} for cosmic strings). We additionally carry out a Bayesian model
comparison of several models with and without defects, showing models with
defects are neither conclusively favoured nor disfavoured at present.
