  Dynamical calculations are performed for all members of the flavor
antidecuplet to which the pentaquark $\Theta^+$ belongs. The framework is a
constituent quark model where the short-range interaction has a flavor-spin
structure. From symmetry considerations the lowest state acquires a positive
parity. By fitting the mass of $\Theta^+$ of minimal content $uudd\bar{s}$, the
mass of $\Xi^{--}$, of minimal content $ddss \bar{u}$, is predicted to be
approximately 1960 MeV. It is shown that the octet and antidecuplet states with
the same quantum numbers mix ideally due to SU(3)$_F$ breaking.
