Net-spinning caddisflies are an important group of filter feeders in many streams, but the mechanisms by which their nets capture particles are poorly understood. Our objective was to determine the effects of particle size on the efficiency of particle capture by 2 species of net-spinning caddisflies, Ceratopsyche morosa and C. sparna. We measured particle capture by nets and ingestion by larvae. We used a mathematical model that incorporated drag and the presence of a near-bed boundary layer to predict reduction of flow passing through a caddisfly net. We evaluated model predictions by measuring the velocity of flow through a scaled model of a C. sparna net in a laboratory flume. Velocity of water through the net was reduced 45% at free-stream velocities of 71 cm/s and 36% at 99 cm/s. We released known quantities of 3 sized particles (Artemia nauplii, 528 μm; corn pollen, 85 μm; paper mulberry pollen, 12 μm) in several stream riffles to determine whether size-selective capture and ingestion occurred. Both C. morosa and C. sparna nets captured particles primarily by sieving; particles smaller than the mesh size (paper mulberry pollen) were caught at 28× lower efficiencies than sievable particles by C. morosa nets and 2.2× lower efficiencies by C. sparna nets. Larvae ingested 73 to 83% of the largest particles captured by nets, compared to 4 to 6% for the smallest particles. The overall probability of particle capture and ingestion of the largest, sievable particles was ∼400× greater than for the smallest, nonsievable particles, with the difference much greater for C. morosa than for C. sparna. We compared actual rates of particle capture to theoretical predictions. Ceratopsyche morosa captured sievable particles at 34% of the predicted rate, whereas nonsievable particles were captured at 0.4% of the predicted rate. Ceratopsyche sparna showed less difference between capture mechanisms, and captured sievable particles at 5% and nonsievable particles at 1.1% of predicted rates. Natural seston concentrations were skewed in the direction of smaller particles; the smallest size class, 0.5 to 30 μm, made up 65% of the total ash-free dry mass of the seston. Large particles (>65 μm) were captured at higher efficiencies by nets and ingested more readily by larvae; therefore, we predict that they will be the primary food items of 5^th-instar larvae of both species studied despite their rarity in the water column.