The effect of stocking density and feed type on the development of the gonadal of Ben Tre hard clam (Meretrix lyrata Sowerby 1851) was carried out in two experiments. The experiment 1: the brooders of hard clam were conditioned in nine 6-m2 tanks for a 21 days period with three different feed: (TNl) used of the mixed of 2 micro algae Nanochloropsis occulata and Chaetoceros calcitrans, with the ratio of 1:1
(TN2) used of blooming algae which fertilized in pond
and (TN3) used of blooming algae in pond in combined with two microalgae N. occulata and C. calcitrans. The experiment 2: the brooders of hard clam were cultured in nine l00-m2 at the densities of 50, 150 and 250 clam/m2
used of blooming algae in combination with the mixed of two microalgae N. occulata, C. calcitran, with 2-3x10 exponent 9 cells/clam/day. The results indicated that the clam brooders cultured at these densities: 50, 150 and 250 clams/m2were no significant difference in survival rate (P0.05). There were also no significant difference in the maturation ratio between the clam cultured at the density of 50 clams/m2 and 150 clams/m2 with 80.00 +/- 6.67 percent and 73.33 +/- 6.67 percent, respectively (P0.05), but higher significantly compared with clams cultured in the density of 250 clams/m2 (62.22 +/- 3.85 percent) (p0.05). The was no significant diference in gonadal development ratio at 3 stage between clams fed by NT2 feed and NT3 were 73.33 +/- 6.67 percent and 80.00 +/- 6.67 percent, respectively (p0.01), but higher significantly compared with the clams fed by NT1 feed (64.45 +/- 6.65 percent) (p0.05). Thus, it can conclude that the suitable stocking density for the brooder clams for the conditioning was 150 clams/m2, while natural blooming algae in combination with two microalgae: N. occulata and C. calcitrans were also suitable for feeding brooder clams during the conditioning period in large production scale.