This used to stump me: is series singular or plural?
Series can be singular or plural–and it’ll sound the same: series. (Same thing with the word species) Series comes from Latin (meaning “chain”), and the word remains the same whether singular or plural.
The Encarta Dictionary was very helpful on this subject (alas, I can no longer find Encarta Dictionary’s explanation online):
Series can be a singular or a plural noun, depending on its meaning. When it is used to refer to a single set of things, it takes a singular verb even if it is followed by the preposition of and a plural noun.
Example: A series of medical tests is planned for next week.
When series refers to two or more sets of things, it takes a plural verb:
Example: Three series of medical tests are planned for next week.
The American Heritage Dictionary (cited in Free Dictionary) explains:
Series is both a singular and a plural form. When it has the singular sense of “one set,” it takes a singular verb, even when series is followed by of and a plural noun: A series of lectures is scheduled. When it has the plural sense of “two or more sets,” it takes a plural verb: Two series of lectures are scheduled: one for experts and one for laypeople.