Clusters in the Luminous Giant HiiRegions in M101

(Abridged) We have obtained HST WFPC2 observations of three very luminous butmorphologically different giant HII regions (GHRs) in M101, NGC5461, NGC5462,and NGC5471, in order to study cluster formation in GHRs. The measured (M_F547M- M_F675W) colors and M_F547M magnitudes are used to determine the ages andmasses of the cluster candidates with M_F547M <= -9.0. NGC5461 is dominated bya very luminous core, and has been suggested to host a super-star cluster(SSC). Our observations show that it contains three R136-class clusterssuperposed on a bright stellar background in a small region. This tight groupof clusters may dynamically evolve into an SSC in the future, and may appearunresolved and be identified as an SSC at large distances, but at presentNGC5461 has no SSCs. NGC5462 has loosely distributed HII regions and clusterswithout a prominent core. It has the largest number of cluster candidates amongthe three GHRs, but most of them are faint and older than 10 Myr. NGC5471 hasmultiple bright HII regions, and contains a large number of faint clustersyounger than 5 Myr. Two of the clusters in NGC5471 are older than R136, butjust as luminous; they may be the most massive clusters in the three GHRs. Thefraction of stars formed in massive clusters is estimated from the clusters'contribution to the total stellar continuum emission and a comparison of theionizing power of the clusters to the ionizing requirement of the associatedHII regions. Both estimates show that <~ 50% of massive stars are formed inmassive clusters. The cluster luminosity functions (CLFs) of the three GHRsshow different slopes. NGC5462 has the steepest CLF and the most looselydistributed interstellar gas, qualitatively consistent with the hypothesis thatmassive clusters are formed in high-pressure interstellar environments.