Zotero can import from various bibliographic formats:

• Zotero RDF, Bibliontology RDF
• MODS (Metadata Object Description Schema)
• BibTeX (rich format but lacks standardization)
• CSL JSON
• Endnote XML (this is actually very similar to RIS in importing from Endnote, but may have some small advantages and is one of the few styles that will preserve italics across imports.)
• MAB2, MARC, MARCXML
• MEDLINE/nbib
• OVID Tagged
• PubMed XML
• RIS (this can be convenient for quick edits between export & import because of its simple structure)
• RefWorks Tagged (recommend for ReferenceWorks)
• Web of Science Tagged
• Refer/BibIX (discourage if any other option present)
• XML ContextObject
• Unqualified Dublin Core RDF