The widely used collective pinning approach is applied to describe the crit
ical current density J(c) of YBa2Cu3O7-delta single crystals. The field dep
endence of J(c) at low temperatures, before and after reactor neutron irrad
iation and annealing, points to strong pinning. Collective effects are expe
cted at high densities of pinning centers, but the description is not possi
ble in terms of the standard weak pinning model. A comparison of the data o
f untreated and neutron irradiated crystals indicates that strong pinning a
lso prevails in the as-grown material. Further results on related single cr
ystal samples (RE-123 and Y-124) support the findings.