Online Pennsylvania Naturalization Records & Indexes

Pennsylvania Naturalization Records Online at Footnote (searches are free; requires payment for copies of the records)
these include scanned images from the National Archives microfilm - searchable by name (note: these databases are not yet complete, but more records are being added to them)

Philadelphia Area: Naturalization Petitions for the Eastern District of Pennsylvania, 1795-1930 about 83% complete (from National Archives publication M1522)
Mid-PA Area: Naturalization Petitions of the U.S. Circuit and District Courts for the Middle District of Pennsylvania, 1906-1930 about 99% complete; also includes US Circuit Court records for 1901-1906; for courts in Scranton, Williamsport and Harrisburg (from M1626)
Pittsburgh Area: Naturalization Petitions of the U.S. District Court, 1820-1930, and Circuit Court, 1820-1911, for the Western District of Pennsylvania about 94% complete (from M1537)

Pennsylvania Naturalization Indexes Online at Ancestry (these require payment)

County Listings

Centre County Naturalization Records 1802-1929 includes digitized images of the records
Chester County Naturalization Records Index 1798-1935
Lackawanna County: US District Court, Scranton Pennsylvania Declarations of Intention Index 1906-1908
Tioga County: Abstracts of Naturalization Records (Tioga County Courthouse, Wellsboro, PA)
York County Naturalization Index c.1799 to c.1990

Finding Aid for Philadelphia Naturalizations

Pennsylvania: Researching Philadelphia Naturalization Records