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We present HST WFPC2 observations, supplemented by ground-based H-alpha data,of the star-cluster populations in two pairs of interacting galaxies, selectedfor being in very different kinds of encounters seen at different stages.Dynamical information and n-body simulations provide the details of encountergeometry, mass ratio, and timing. In NGC 5752/4, we see a weak encounter wellpast closest approach, after about 2.5x10^8 years. The large spiral NGC 5754has a normal population of disk clusters, while the companion NGC 5752 exhibitsa rich population of luminous clusters with a flatter luminosity function. Thestrong, ongoing encounter in NGC 6621/2, seen about 10^8 years past closestapproach between roughly equal-mass galaxies, has produced a rich population ofluminous clusters, particularly young and bright in a small region between thenuclei. This region is dynamically interesting, so strongly perturbed that therotation curve reverses sign. Cluster formation requires a threshold level ofperturbation, with stage of the interaction less important. The location of themost active star formation in NGC 6621/2 draws attention to a possible role forthe Toomre stability threshold in shaping star formation in interactinggalaxies. The rich cluster populations in NGC 5752 and 6621 show that directcontact between gas-rich galaxy disks is not a requirement to form luminousclusters, and that they can be triggered by processes happening within a singlegalaxy disk (albeit triggered by external perturbations). (Abridged)Comment: In press, Astron. J., Sept. 200

Massive Star Clusters in Ongoing Galaxy Interactions: Clues to Cluster Formation