Regular English nouns end their plural forms with an -s or -es, so it's natural to assume the plural noun is “flys.” However, “fly” (pronounced 'flī') is an irregular noun, so its plural form replaces the -y with -ies: “flies.”

Do you say flys or flies?

In general, flies is the only correct version of this word today. If you were writing unusually detailed period fiction about 19th-century Great Britain, you could use flys as a plural noun to refer to a specific mode of transportation.

How do you make fly plural?

noun, plural flies.