In “The Flea,” Donne employs the unusual strategy of alternating meters. That is, the lines in each of the poem’s three stanzas oscillate between iambic tetrameter and iambic pentameter. This means that the underlying rhythm of the poem is defined by the iamb, which consists of an unstressed syllable followed by a stressed syllable (as in the word “de-ny”). However, the length of each line alternates between four and five iambic feet. As an example, consider the poem’s opening stanza (lines 1–9):

     Mark but / this flea, / and mark / in this               [tetrameter]
     How litt- / le that / which thou / de-niest / me is;            [pentameter]
     It sucked / me first, / and now / sucks thee,            [tetrameter]
     And in / this flea / our two / bloods ming- / led be       [pentameter]
     Thou know’st / that this / can-not / be said            [tetrameter]
     A sin, / nor shame, / nor loss / of maid- / en-head,        [pentameter]
         Yet this / en-joys / be-fore / it woo,                [tetrameter]
         And pam- / pered swells / with one / blood made / of two   [pentameter]
         And this, / a-las, / is more / than we / would do.        [pentameter]

First, note the consistency of the iambic rhythm. Other than the substitution of a trochee (stressed–unstressed) in the first foot of the opening line, Donne maintains the iambic rhythm throughout. Admittedly, there are moments where the reader may have to force the rhythm slightly. For example, we have to elide the last two syllables of “de-ni-est” to make “de-niest” (line 2). Similarly, some might read the phrase “bloods mingled be” as involving a spondee, which is a foot with two stressed syllables: “bloods ming- / led be.” However, these variations aren’t insistent enough to break the overall impression of iambic rhythm.

The consistency of the iambic rhythm gains its significance in relation to the inconsistency of the individual line lengths. As mentioned above, each line in the poem alternates between tetrameter and pentameter, creating the following pattern for each stanza: 4–5–4–5–4–5–4–5–5. This alternation produces an overall sense of imbalance that mirrors the lack of consensus between the speaker and his mistress. Despite sharing the same general rhythm, the lines are out of step. In a similar way, the speaker and his mistress both seem tempted by sex, but they aren’t equally ready to engage in the act. The alternation between tetrameter and pentameter formally reflects this lack of agreement. Yet it’s important to note how each stanza has an imbalanced distribution of tetrameter and pentameter lines. That is, each stanza ends with two pentameter lines in succession. Pentameter therefore has a slight edge over the tetrameter. Does that mean that speaker’s case for making love will ultimately win out?