Abstract : The masses of various pentaquarks $uudd\bar q$ are calculated within the framework of a semirelativistic effective QCD Hamiltonian, using a diquark picture. This approximation allows a correct treatment of the confinement, assumed here to be similar to a Y-junction. With only color antitriplet diquarks, masses of the $\Theta$ and similar pentaquark candidates with positive parity were found around 2.2 GeV. It is shown that, if a color sextet diquark is present, the lowest $uudd\bar q$ pentaquarks are characterized by much smaller masses with a negative parity. Masses below 1.7 GeV are computed, if the masses of the color antitriplet and color sextet diquarks are taken similar.