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During the period January 1972–June 1974, Mt Ngauruhoe, an andesitic volcano in the Central North Island of New Zealand, has had 12 main eruptive periods. The fortnightly component of the earth tide is strongly correlated with these eruptive periods. Seven start within 1.5 days of a tidal maximum. The distribution of the date of commencement of these eruptive periods about the maximum of the earth tide is skewed; more eruptions occur before the maximum than after it. Only one eruptive period occurs within 1.5 days of a tidal minimum. The triggering near the time of the tidal maximum may result from the enhanced effects of tidal compression and expansion of the earth's crust on magma beneath the volcano. Compression would cause the magma to rise in the conduit; expansion would promote degassing.