Host-plant recognition in the onion fly, Delia antiqua (Meigen), was investigated by presenting reproductively mature females with surrogate plants and manipulating single surrogate features against a backdrop of unchanged stimuli. Colour, shape, and size of surrogate foliage all influenced oviposition. Releases of dipropyl disulphide, the major volatile component of onion, from surrogate foliage stimulated four times more oviposition than releases from the sand substrate. Thus, placement as well as concentration of host chemicals influenced ovipositional behaviour. Interactions among chemical and non-chemical host stimuli are critical to the host-recognition process. Removal of a chemical stimulus or alterations of foliage colour or size caused similar reductions in number of eggs laid. Observations of females alighting on stimulatory and non-stimulatory onion surrogates indicated that colour, shape, size, and chemical stimuli influenced both alighting and post-alighting behaviours. However, post-alighting behaviours, such as running over foliar surfaces and ovipositor probing, were most highly correlated with egg numbers; surrogates that stimulated these behaviours also stimulated the most egg laying. Effects of altering host-plant stimuli were also examined in greater detail by quantifying egg production of individual females given a single surrogate type over an 8-day period. This no-choice test quantified specificity of onion fly ovipositional responses. Elapsed times until 50% of the females accepted oviposition sites were about 7.5, 7.8, 8.8, and > 13 days for unaltered, chemically altered, colour-altered, and size-altered surrogates, respectively. Temporal distributions of egg laying differed for females given altered and unaltered surrogates. Whereas females on unaltered surrogates distributed their egg laying evenly over the experimental period, females on altered surrogates laid a significantly larger portion of their eggs 4 days into the experiment. This temporal concentration of egg laying coincided with maximal accumulation of mature eggs in the ovaries of deprived females. In summary, we have not found good evidence for hegemony of chemical stimuli in the sensory world of D. antiqua. Although single host-plant chemicals are sufficiently stimulatory to elicit oviposition in deprived females, the same can be said for structural and colour cues from onion foliage. In undeprived females, host-colonization is most effectively stimulated by the integration of chemical and non-chemical subsets of the total stimulus pattern represented by onion plants.